That inadvertent shot or set of circumstances that puts the kibosh on what you have been working for.
Based on the games I own:
CFTBL - having the ball go into the bumpers during multiball restart. ARGH!
AFM - trying for 5-way combo and hitting an orbit shot before that feature is completed. CRAP!
MM - hitting Merlin's Magic when only one or two multiballs are lit. RATS!
LOTR - anything involving the right ramp, Witch King or Return of the King multiball, or worse, the first ball dropping out of the ring upon soft impact with the second ball during DTR. NUTS!
WH2O - mistakenly locking the third ball when only one more feature or so would set up VJ. DANG!
What are yours?