My most frustrating capture in White 2 so far has to be Trubbish. It's at yellow health, almost red, and it's been breaking out of balls left and right. Then I throw another Pokeball. It's got the weird sound and the flash that proclaims it to be a critical capture. It's only the second critical capture I've ever seen in my games.
the **** thing breaks out of it. A. Stupid. Trubbish. It doesn't have a low catch rate or anything, and it was in fainting range, but the dumb thing actually broke through a critical capture and managed to KO one of my pokes before I was able to waste a frigging Ultra Ball to capture it for the dex entry.
But I got my revenge. I released it back into the wild. AFTER stealing its precious black sludge. >:-]