Catch good cards when you defeat not let go, thus losing all the chips on hand.
The face of the other bets, holding a weak hand with the note.
Play too many starting hands.
When the hands holding the best hand before the flop did not raise (to hold potential license, applying pressure first bet players), but in too far with them after the flop.
Relative pot bet bet too much / not enough (too risky bet for winning a small amount / their hand unprotected)