1 TILTing. This is easily my biggest leak. Especially when I was once a big stack in an MTT, but after 3 straight suckouts I'm down to average or below average. I'm prone to just giving up and playing recklessly when I still have enough chips to make a comeback.

2 Patience. Another reason I don't do as well in MTT's as I should be. Sometimes I don't get a playable hand in a few orbits, so I get bored, and decide to make a "creative" raise UTG+1 with something like Q-5s...

3 Over-stealing, especially against a short-stack. Shorter stacks are looking to double up, and don't need much to call, but sometimes I'll ignore this and put them all-in with any two (this is a sound play if they are in the "dead zone and have literally nothing left, but I often mis-use it when they're still in high-red zone).

4 Trying to pick off bluffs too much at times, especially over-bets. When it works I look like a genius (I once called a big all-in with K-3off on board of A-8-7-8-A, putting my opponent on a busted straight/flush draw based on the betting patterns, which he did have). But sometimes tricky players are capable of checking a flopped monster two or three times, then overbetting on the end. If I have anything at all I tend to call.

Probably many more, which I can't think of right now.