Giving in to emotions.
Emotional trading is a very bad habit to have and must be conquered. It leads you astray and makes you irrational. You will lose money and this fact compounds the problem. Having a plan, going with the plan and not being tempted to tinker, fiddle and override the plan will win out in the end. In my early trading days I fiddled and tinkered with trades when it started to get a bit hairy - and they have always lost money. I no longer interfere with the plan.