FIRST time I ran Arachnophobia, I succeeded.
First time I ran it with my second AND third characters, they died. It's called overconfidence.
By no means best (or worst) oops moment, but two days ago I approach what's-her-name Mushroom Collector in Harbor. I have 3 whitecaps and 2 deadly feverbranches. Mental note: DO NOT click "Take every set you need"! Whitecaps individually only!
Guess what I clicked...
Oh, here is one from couple weeks ago.
Spent way too much time combing through AH looking for a Disruption weapon. Hardly any, and go outbid on those that there are. Finally find an ML 10 (I am level 10) +1 ghost touch light mace of disruption, with 10k pp buyout. Woohoo!
Take a close look at it after I take it out of the mailbox... it is +1 ghost touch light mace of destruction
Funny thing, it turned out to be actually more useful than plain ML 10, DC 14, +1 mace of disruption (which by now I also have).