[Lipe82]

I'm about to run my first SR5 game.
What aspects of the game the more experienced GMs would tell me to look in advance for its chance to screw the session?

Troublesome rules, unbalanced powers (for better or for worse), inconsistencies and the like - let me know.

What kind of things you tend to rule out/houserule to avoid trouble?
Help a new Mr. Johnson out.

[Prime Mover]

Don’t get bogged down in details.

Your players will find the warts and either exploit them or attempt to interpret them

When in doubt, don’t stop game for to find something, rule on the side of fun and keep up the momentum.

Use your time between sessions to brush up on the details and clarify them next session.

Bookmarks can be great help and find cheap colored adhesive ones pretty easy.  When find yourself returning to specific chart or rule, mark it for quick reference.

[Prime Mover]

Everyone’s different and finding counters or balancing encounter's in Shadowrun are easier after you see how your players approach them. There are powers or abilities that can seem imbalencing or used in ways that seem like instant wins.  But remember you can throw those same things or even worse right back at them for a tense encounter.

[BeCareful]

If you haven't done character creation yet, encourage everyone to toss ideas around together. Help them through it, with stuff like putting chargen skill points in "gun", then skimming the gun list before narrowing it down. Then tell them to give you backstory/Quality details in private.

If you have done chargen, then all I have to say is, if the players find something you hadn't thought of, some kind of slip-up in your design, either ad-hoc something in their that further research would uncover, or congratulate them on finally figuring out what you totally had planned the entire time. Then give it an in-setting justification for why something essential was left out. Null sheen, and such.

[Ixal]

I still remember the disaster that was my first session. We all came from D&D and thus many players still had a very D&D mindset which did not work out very well. Things like the mage not wearing armor out of habit or thinking they have to "crawl" through a corporate office and explore everything.

So if you too come from an other RPG then take care to manage expectations and also allow "rewinds" until you players unlearned their previous RPG.

[Sphinx]

SESSION ZERO. Before you get around to building characters and rolling dice, make sure you sit down with all the players together and talk about your campaign, just to make sure everyone is on the same page about things like:

• The scope of the campaign: Will events unfold inside a single arcology, a local neighborhood or district, a major sprawl, a country, or worldwide? Players ought to know before they design their characters so they don't invest too much in resources they'll never be able to use (e.g., tricked-out vehicles, extensive local contacts, home ground qualities).
• The time frame: How much downtime do you expect between shadowruns? Days, weeks, months? This affects things like Edge refresh, skill training, initiation, cyberware surgeries, and lifestyle costs.
• Rewards: What kind of nuyen and karma awards can players expect, and how often? Will you allow cash-for-karma/karma-for-cash conversions? Is there "extra credit" for players who keep journals/blogs/chronicles, handle necessary bookkeeping, read/report on new sourcebooks, or bring snacks? Some character concepts depend on karma for improvement (magicians, adepts), others need money for gear and augmentations (riggers, samurai). Players might get frustrated if other characters advance faster than theirs because they chose the wrong direction.
• Professional ethics: What are the expectations for professional conduct among shadowrunners in the game? Not that players are necessarily bound to follow them, but they ought to know what they are before they cross any lines.
• Character morality: What's acceptable conduct for player characters in the game? On a sliding scale between villain and hero, what are the boundaries? Be clear about the difference between player sensibilities vs. character sensibilities. 
• Player issues: Do any players have pet-peeves or off-limits areas or triggers that everyone needs to respect? (e.g., torture, sexual assault, substance abuse, mental health, violence against children, foul language). Shadowruns can get pretty dark sometimes; make sure everyone understands what's acceptable for other players at the table.
• Bookkeeping: Are you going to keep track of karma and nuyen rewards and expenditures, or are players going to handle that themselves? Google spreadsheets are your friend.
• House rules: Are there any house rules or optional rules you want to start with? What's the process for adopting new ones? GM fiat, majority vote, unanimous vote?
• Table rules: Do dice count if they fall off the table? After a player moves their token on the map, can they change their mind? Is idle surfing on phones, tablets, or laptops acceptable at the table when the spotlight is on other players?
• Rules lawyering: How much is allowed? Some groups have as much fun discussing rules as rolling dice; others not so much. If players know ahead of time that there's a time limit, they're less likely to get angry later when you shut it down and move on. You can always debate the call after the game, or by email between sessions.
• Q&A: Players may have questions or concerns you haven't considered. Make sure everyone gets a say.

[FastJack]

SESSION ZERO. Before you get around to building characters and rolling dice, make sure you sit down with all the players together and talk about your campaign, just to make sure everyone is on the same page about things like:

• The scope of the campaign: Will events unfold inside a single arcology, a local neighborhood or district, a major sprawl, a country, or worldwide? Players ought to know before they design their characters so they don't invest too much in resources they'll never be able to use (e.g., tricked-out vehicles, extensive local contacts, home ground qualities).
• The time frame: How much downtime do you expect between shadowruns? Days, weeks, months? This affects things like Edge refresh, skill training, initiation, cyberware surgeries, and lifestyle costs.
• Rewards: What kind of nuyen and karma awards can players expect, and how often? Will you allow cash-for-karma/karma-for-cash conversions? Is there "extra credit" for players who keep journals/blogs/chronicles, handle necessary bookkeeping, read/report on new sourcebooks, or bring snacks? Some character concepts depend on karma for improvement (magicians, adepts), others need money for gear and augmentations (riggers, samurai). Players might get frustrated if other characters advance faster than theirs because they chose the wrong direction.
• Professional ethics: What are the expectations for professional conduct among shadowrunners in the game? Not that players are necessarily bound to follow them, but they ought to know what they are before they cross any lines.
• Character morality: What's acceptable conduct for player characters in the game? On a sliding scale between villain and hero, what are the boundaries? Be clear about the difference between player sensibilities vs. character sensibilities. 
• Player issues: Do any players have pet-peeves or off-limits areas or triggers that everyone needs to respect? (e.g., torture, sexual assault, substance abuse, mental health, violence against children, foul language). Shadowruns can get pretty dark sometimes; make sure everyone understands what's acceptable for other players at the table.
• Bookkeeping: Are you going to keep track of karma and nuyen rewards and expenditures, or are players going to handle that themselves? Google spreadsheets are your friend.
• House rules: Are there any house rules or optional rules you want to start with? What's the process for adopting new ones? GM fiat, majority vote, unanimous vote?
• Table rules: Do dice count if they fall off the table? After a player moves their token on the map, can they change their mind? Is idle surfing on phones, tablets, or laptops acceptable at the table when the spotlight is on other players?
• Rules lawyering: How much is allowed? Some groups have as much fun discussing rules as rolling dice; others not so much. If players know ahead of time that there's a time limit, they're less likely to get angry later when you shut it down and move on. You can always debate the call after the game, or by email between sessions.
• Q&A: Players may have questions or concerns you haven't considered. Make sure everyone gets a say.

+1

I try to do this for every game I run.

[Marcus]

So while there are holes in 5e, but there is a LOT of 5e, and your players may or may not ever run across them, most are specific corner cases a specific power or piece of gear. To me starting a new group into 5e is just the volume of material to get the full picture is big, that may not matter to your group, there isn't a right or wrong answer to that. But it is something to be aware of. Certain things like TM and Alchemist have change a lot over the course of the edition. So if you have players going down those paths, make sure your current with each of those.

[Sphinx]

COMBAT. Running fights in Shadowrun can take awhile, especially for new players. A few tips for keeping combats from dragging:

• Vital NPC stats: When you expect a fight, make a combat worksheet ahead of time, recording the most important stats for each NPC: attack dice pool (skill + Agility) and weapon damage code, defense pool (Reaction + Intuition), and damage resistance (Body + Armor), plus modifiers. It helps to roll NPC initiatives in advance, especially if there are lots of them.
• Vital PC stats: Make sure your players (especially inexperienced ones) have these same stats highlighted on their character sheets, or penciled at the top of the page, or written on a sticky note. Attack, defense, soak. You should know them, too.
• Environment modifiers: Something else for your combat worksheet: a quick summary of any environment modifiers (p.175) for light, visibility, etc., as well as Matrix noise or astral background count.
• NPC tactics: Try to work out ahead of time what the NPC tactics will be, what actions an enemy hacker will take, what spells the magician will use, etc. Note relevant dice pools and page numbers. Bookmark important pages in the rulebook with yellow sticky notes. Don't hesitate to change tactics in response to player actions, but the fewer decisions you need to make in the middle of a fight, the faster things will go.
• Player focus: It's easy for players to get distracted and lose track of the initiative order, especially in larger groups. Then they're surprised when their turn happens and they waste time deciding what to do. Try to warn each player when their turn is about to come up. ("Ann, it's your action. Bob, you'll go after Ann.") When their turn happens, they should already know what their action will be, and have their dice counted and ready to roll. If they dither, have them delay their action (p.161) until after the next player's turn.
• Player minions: If one player has multiple minions (drones, bound spirits, registered sprites) with their own initiatives, that player can end up monopolizing more than their fair share of playtime in combat. Have them write stats on index cards and pass them out to other players around the table. They still give the commands, but the other players can interpret and carry them out.

[Michael Chandra]

Dice: You'll be rolling more than your players. Use a few different colors, preferably in fixed amounts (each type 6 or 9 I'd say) so you can quickly gather a dicepool without full counting.