When Stanley discovers Munson has been gambling with a false wad of cash, he says "What's this? A hundred dollar bill wrapped in Monopoly money?"
Actually he should have said: "Monopoly money wrapped in a hundred dollar bill?"
When McCracken pulls off Roy's fake hand no hook is visible. Roy sits in the chair and wipes his head with a towel then puts towel in his lap and clearly puts the hook in his right hand using the towel as a cover. When the towel is removed the hook is now visible.
When the strip club owner is about to beat Ishmael with a bat, the cigar in his mouth disappears. And in the same scene Ishmael's hat falls to the ground when he's pushed up against the wall but a few seconds it re-appears on his shoulder.
When Roy is young and wins the State Championship, he gets a new gold Oldsmobile. The taillights are 3-segimented horizontally as in 1972. Later, when Roy and his car are older, the taillights are non-segmented vertical lights as in 1971.
Right before Roy meets Ishmael in the bowling alley for the first time, he rips his rearview mirror off his car with his hook. After he leaves the bowling alley to catch up with Ishmael, the mirror is re-attached.
Despite the popular urban legend, putting sugar into a gas tank will not ruin a car's engine (although it is very inconvenient). Sugar does not dissolve in gasoline, so although the crystals could potentially clog the fuel filters or injectors (requiring both them and the tank to be cleaned) it doesn't cause the massive damage that the legend suggests.
At the start of 10th frame of the final match, the announcer says that 3 strikes (30 points) would give Roy "a nice cushion". After Roy only earns 20 points, we are told he then has a 29 point lead over Ern, forcing Ern to score the maximum 30 points to win. Had Roy scored 30 points, he would have had a 39 point lead (more than Ern could possibly score). That's not a "nice cushion"; that's a victory.
During the Reno tournament, there are several instances where you can see the scoreboard above the lanes. Each time, the scoreboard has multiple names under each of the lane headings despite the fact that only one bowler is on each lane.
During the final match with Big Ern, Roy bowls both the 1st and final frame on the right lane. During tournament play, bowlers will alternate lanes each frame meaning he should finish the game on the opposite lane. (Also, they do show Roy bowling on the left lane during the match, meaning they must be following the alternating lane format).
When Roy's rubber hand has been removed and the hook is visible, his right arm/sleeve is suddenly longer than his left (in fact, it's just longer enough to accommodate the hook that Harrelson is holding in his hand at the time). This is most obvious at the beginning of the tournament scene, when Roy's hand falls off with the ball. His right arm is suddenly a several inches longer than his left, not counting the hook, but when he puts the hand back on his arms are the same length again.
When Roy first meets Ishmael in the bowling alley, he tells him to walk two floor boards to the left. When Ishmael rolls the ball, a stick is visible hitting the pins to ensure that Ishmael gets a strike.
During the Reno tournament, there is a view of one of Roy's strikes from behind the pins. When the pins go down, the ball is still visible having stopped by the pins instead of continuing to roll off the end of the alley.