1) Buying an extra Sixaxis... I barely touched it, then the Dual Shock 3 came (I didn't know about it)
2) Buying Pixeljunk Monsters right before it goes on sale for $5 (Yeah, it's only $5, I know)
3) Buying Resistance: FoM. With all the good reviews, I expected more than just a generic shooter. I'm not a big shooters fan but honestly can't see what's good about the game.
4) This goes with #3 - buying the PS3 before it dropped $100. If Resistance was the shooter I expected (like CoD4, which is great), I wouldn't be so annoyed, but damn.
5) Never having touched the MGS series. I missed the PS1 era (the N64 / PC kept me busy), and I never bothered to revisit the game. Still need to work on that...