My biggest AH screwup was back in the vanilla WOW days. I was an enchanter, I was one of the guild's main enchanters that DE'ed stuff from our raids. We had an excess of Nexus Crystals, which at the time went for 90g a piece. I accidentally listed 60 of them at 90s a piece!

When I got the mails from the AH I immediately realized "oh !%#@", some lucky fool got a huge windfall buying and relisting those.

Back then losing 89g x60 was a huge deal, people didn't run around with 1000's of gold in their pocket. There were no dailies to make epic gold from, you had to farm!