1. Overvaluing weak Aces in EP: I see people lose money with these continuously. Regardless of whether they raised or limped in pre, if an Ace hits the board they wind up getting stacked when somebody else has a better kicker.
2. Small pocket pairs, especially in EP: Playing these like AA/KK is a good way to lose all your chips fast. Overcards be damned, it's full speed ahead? Not a good strategy.
3. Good old AK: I had more trouble with this hand than any other when I was starting out. I've eliminated most of my losses with AK, once I broke the habit of spewing chips post-flop with just Ace high.
4. Paying too much to chase draws aka ignoring pot odds
: People remember the times that they called an all-in on the flop or turn and hit that flush or gutshot straight on the river, but don't realize that they're losing much more than they gain in the long run by doing this.
5. Double standards: viewing hands where you put your money in way behind but sucked out massively on the river as skillful plays, but viewing hands where your opponent did the same to you as the product of either rigged software or some other nefarious plot to inflict you with undeserved bad beats.