I haven't... but I know quite a few people who have (they were all 8 and under when it happened)
Tell your stories! And then moarn the deaths... *sniff*
Here are my friends' stories:
One wanted to warm up her hamster, so she put it in the microwave... and... well... let's just say that she had a big mess to clean up afterwards. She moarned him for many days.
Another's cat had kittens, and apparently her little brother (3 at the time) decided that since he was cleaning the windows near them, that he was going to clean the cats. With Windex. Two out of the three died soon afterwards.
Before people begin saying that it's cruel, and that parents should educate their kids better, or at least keep a closer eye on them. Both of these times the parents were out of the house (or out of sight), and there are SO many ways that you can accidentaly kill a small animal, that... well... it's impossible to warn your three or four year old against ALL of them. :P