1.) Installed the client some time around December 2010. Set it to "mine". It made my laptop go loud and hot. Nothing else happened for the 30 seconds my patience lasted. Deinstalled it right away thinking there was no point to it at all. Had I only started GPU mining right away... it was hilariously profitable back then.

Took me several month to randomly bump into Bitcoin again and finally understand it's implications.

2.) GLBSE
Peter Lambert and his LIF stuff.
Some other company that I really deserved to loose on.
Zipconf (Still think it was a good idea)

3.) Not selling a single coin in the 2011 bubble.
To buy back at lower prices. Hmm, this one seems to remind me of something............