Deliberate Johnson screws should be very, very remote. The problem is that it's a trope, so eveyrone wants to use it, until it became virtually the norm, and that's bad for Shadowrunning.
Now, accidental ones? That's fine. "You said there'd be light security! My team got shot by cybered-up goons!"
"Oh. Sorry about that. Looks like I wasn't informed of the personell transfer."
"... Fragger! Alright, we still got your chip, but the price just went up to cover my guys' medical bills."
A little oopsie is fine and dandy, but taking Johnson away, and the money he was paying, just cripples the trust of a group. Use it sparingly if at all.
(Of course, I once had a team come back to find Johnson was just *gone* ... while they were on a mission, a corporate team had gotten wind of his activity and swooped in to pick him up for 'debriefing'. They were able to turn around and sell their loot on the open market, so it wasn't a total loss, but, they didn't get as much as Mr J was offering.