I've run a number of campaigns over the years, and I like writing up the slow-build-up-to-Oh-Sh*t moments shadowruns. For example:
1) The runners are investigating a spat of increased gang violence. Tracking the guns that one of the gangs were using (very high-powered Russian guns), the group discovers the guns were purchased and sent to Seattle by the owners of a small ambulance service... a company that has a city contract to pick up the dead bodies at crime scenes. Investigating the ambulance company, the group spies the owners (a group of intelligent ghouls) tasting the bodies of the dead gangers, labling them as "NO" and "Yes, it's another one". Asking "Another one of WHAT?" the group steals the body and has the toughest mage they could find assess it... and discover from black streaks in it's rapidly fading aura that the body is of a cannibal (an eater of human flesh). Confronting the ghouls, the group learns that their leader (a ghoul voodoo priest) is at war with a wendigo that is working to take over the city (I had the house rule that wendigos can only eat the flesh of other cannibals, and that their Influence power is magnified greatly when used against cannibals, so wendigos work very hard to trick or force others into eating human flesh). The wendigo had been spreading it's influence through the street gangs of the city, and was now preparing to host a dinner party for the movers and shakers of Seattle society. The party, backed by the ghouls, had to crash the biggest social event in Seattle and kill it's host before the entire city fell under the wendigo's sway.
2) Group is hired by a Yakuza boss to sneak into a Mallplex (a low-end archology in which several corp-owned tenement buildings are attached to an enclosed mall, and the comings and goings of the residents are controlled by implanted Rfid chips and security gates) in order to break out the family of an up-and-coming singer that is under contract to one of his rivals. The Yak tells them that if he can get the family out of the Mallplex's control he would break his rival's hold on the singer, and she would be free to sign with his label. What the party doesn't know is that the Yak already has support in his fight against his rival... a corp strike team (that the party had crossed before) which is supporting the Yak in order to have leaverage over him when he gains more power. The party is the strike team's payment for their latest assistance... they asked the Yak to send the party into a high-security zone, unarmed, where the striketeam could handle them at their leisure. Fortunately, two things happened in the party's favor... the party's cat shaman got bored and hung out invisibly under the Yak's table after the party had left, and heard the Yak call his corp contact, and the Yak's rival (another Yak) kidnapped the kid sister of another party member in order to blackmail him into betraying his rival. Things got interesting after that...