Continuity mistake: After Willie and Marcus rob the first safe, they are sitting at a bar. Marcus agrees to one more drink. They tap glasses and Marcus takes a drink and leaves it half full. Next shot its empty. Next shot there's just a tiny amount in the glass.
Continuity mistake: When Willie gets thrown over the bar, two stools fall over. In the next shot, they are back up.
Continuity mistake: The washcloth changes color and design when Willy is in the tub.
Continuity mistake: In the kitchen scene after Thurman cuts his hand, Willie turns off the faucet and then it's back on again.
Continuity mistake: When the kid takes Willie the orange juice there is sun on Willie's face. Camera cuts and it's disappeared.
Answer: My guess is that when he was a child, he was molested by a man dressed as Santa Claus, and ever since then he has been holding a grudge against people dressed as Santa Claus. You can see him staring angrily at Willie in the bar even before Willie notices him.