1. Playing in a game too big for your bankroll.
2. Playing when your mind is affected by outside influences.
3. Not paying due attention to game selection.

These 3 errors are probably the worst you can make in poker and can negate everything else that you do right and the really horrible thing about these errors is that you need only make them the once for disaster to strike and this is the trouble with them, these are not errors you can get away with every now and then, these are errors which you must strive to never ever make.

This is the thing though no matter how well we play, all it takes is for some horrible decision making for things to go major wrong. The true horrible decision making I believe is not actually made in the actual table you are sitting in whether you bet, fold, call, raise etc, it is done when logging in to a poker room and picking a game to join, before you ever boot up that PC you need to have your battle plan.

It can be adapted later but have your mindset correct in the first place, unlike in real war where sometimes you don't have any choice, in poker you do have the choice. If you are a wounded animal don't engage in battle, i.e. after a heavy night on the booze or an argument with your partner, spouse forget about playing poker REPEAT FORGET ABOUT PLAYING POKER.

It's not that playing under these conditions in your usual game is bad enough, quite often you'll be even more stupid as you are not thinking straight and compound your initial error and play in the very kind of games which you would normally avoid, e.g. the ones where you're bankroll can't stand, or games where your opponents are real tough or even when you join a weak table and then tough players join you don't head for greener pastures like you might do if in your normal frame of mind.