As the topic says, What are the biggest mistakes you've done with Bitcoin?

I'm trying to gather just a list on things that have gone wrong. This can be terrible backup stories, client hacks, reversing at MtGox and such.
I don't want to gather a list of hacks of commercial sites in particular because there are plenty of information on that already.

I will post everything I find on this very simple website:
http://sites.google.com/site/whoissatoshi/bitcoin-mistakes


Don't be shy!


EDIT: Please add some context, interesting details etc to make it a nice story.