Leaks, we all have them. I like to think about how much a better player would win (or lose less....which is the same as winning) on any given hand. Poker leaks are like holes in a ship, eventually they will sink the largest one, no matter how small they may seem. Here are some common ones, and some of mine.
1. Calling (or chasing) with insufficient odds. Example: flush draws must get 4:1, straight draws 5:1, gutshots 10:1 (ignoring "implied" odds).
2. Telegraphing moves. For example, reaching for chips....this comes under the category of tells, and is a subject for another day.
3. Not playing with sufficient aggression. It has been said that if you cannot raise a raiser, you should fold......ignoring for a moment position, stack size, the player, other callers, pot size, implied odds, etc. My best poker training early on was playing in a "Raise or Fold" limit online game. Every street was capped and aggression plus good starting card selection was important.
4. Overcalling on the river. I see this all of the time. Big pot, initial raiser bets, more than one caller. Yes, a huge pot it is a mistake to fold often, but most of the time you must be super strong to overcall. This fact is ignored by a lot of players. I would, for instance, overcall with a straight on a paired board or a 3 flush board against some players, but not others. Can you imagine ever doing that if Randy is the original bettor, Evan calls? On the other hand, if it is Aaron & Joe, in a $200 pot, ya sure you betcha!
5. Small blind completion. This one is a tough call. Yes, you are getting great odds in a family pot, and when you are delivered the "small blind special" it is very lucrative. On the other hand, playing garbage in the worst position seldom works out, so I like to limp when it is 4 or more limpers as you are getting 10:1 on your money. Fewer, not so much. The $1. you save every rotation adds up.
6. Not folding to extreme aggression. Let's say you have pocket queens, and raise but are raised, re-raised. With a jack high flop it is raised and re-raised in front of you. Have to pause and ask yourself what they could have that would cause that action. A fold would not be a bad play here, and could save you at least $80.
7. Playing too long, especially when behind and getting rotten cards. We all want to play to get ahead or get even, but stuck is stuck and better to stop digging that hole you found yourself in when you are tired and running bad.
8. Raising with mediocre hands out of position. I see this a lot. Many limpers, small or large blind busts out a fairly large raise (and sometimes a small one!). In my opinion, you better have aces here, yet I see it with hands as weak as A/10. WTF!!!!!
Here is a good link:
http://www.learn2holdem.com/poker-leaks-analysis/poker-leaks.htm