Continuity mistake: In the scene when Worm and Mike go to see Gramma about Worm's debt, Gramma says, "What's the matter, Lester? You looking for a little charity?" Worm curses at Gramma and takes the toothpick out of his mouth and throws it on the floor. Gramma grabs Worm immediately and the toothpick is back in Worm's mouth.
Revealing mistake: The movie is set in New York, but in the final scene you can see a sticker on the door of the law school that says "Smoking in this building" where the bottom reads "State University of New Jersey".
Continuity mistake: When Mike is in the bath talking to Knish, his right hand moves around a lot. Whenever Mike is seen from behind, his hand is on the marble column next to him, but when he is seen from the front, both his hands are in his coat pockets. Back and forth, through the entire scene.
Other mistake: When Mike loses his bankroll to KGB, he's wearing sweaters/jackets, so it's obviously cold out. 9 months later, he is driving a truck for Knish and wearing cold weather attire. It appears to be a month or so before the WSOP, which at the time was held in late April through May, and at the Judge's game, they indicate he may be an intern for one of them during the summer. So at this time it has to be March/April. 9 months earlier would be July/August, no reason to be wearing winter clothes.
Factual error: After Mike sets Worm up at the club and leaves, Worm asks for $2,000 of debt in Mike's name to sit at what was moments before identified as a $20/$40 limit game. The woman is next seen putting four rows of green $25 chips into a chip rack. A $20/$40 game would never be played with $25 chips because this denomination makes it very difficult or impossible to bet the required amount, at least without $5 chips to make change on every bet, which would be extremely tedious. Rather, $20/$40 games are played with $20 chips made specifically for games of those stakes, and they are usually yellow in color.
Suggested correction: Maybe they don't have any of those left. In that case he could play with those.
Other mistake: Mike sits 300/600 limit poker and he wins one pot. The math doesn't add up to 30k.