As title says...
Lets see. Very very first time in Redridge. Back near the first couple weeks of release. Had a human paladin, and was pwning everything with this one ability (forgot what it was called, but it was later nerfed since it was so powerful)...and within the first 6 whelpings I killed.
A little black whelpling pet dropped. I said, "wow stupid drop, only little kids would want a pet that has no use" and deleted it.
I still regret doing that
Still never found a whelpling pet after that...and its been years lol...and I'm too stingy with my money to buy one on the auction house.
I also deleted a level 60 priest...when I wasn't paying attention, had stayed up 2 full nights playing WoW, dead tired...and meant to kill off my low level rogue and well...there went the priest. My guild mates laughed...I only laughed later.
I think the funniest one though was back in wrath of the lich king. Was leveling a warlock, tabbing mobs cause I felt lazy (still nooby as ever)...and accidentally sent my voidwalker two rooms down the dungeon (shadowfang keep). Pulled the voidwalker back realizing my mistake...and in comes 2 rooms of mobs.
The funny part was, EVERYONE blamed the hunter and kicked him from the group.