Reanimated Optimistic Scavenger with the Flashback cost of Rite of the Moth while I had Defiled Crypt out on the field. I didn't get to put a +1/+1 counter on one of my creatures which lost me my game (in a paid draft).

I had reanimated it three times before that (Twice the optimistic Scavenger and once more another creature) and it always worked correctly until the flashback of Rite of Moth (even the normal version went correctly.