Continuity mistake: When the Dude is in Maude's limo, his glass of White Russian is half full. The angle cuts to the driver when he says "Fuggetaboutit." When we cut back to the Dude, his glass is almost completely full.
Continuity mistake: In the scene in the limo, with the Dude drinking his white Russian, some shots show him with a milk moustache, then the shot changes, and then when it returns he doesn't have a moustache. The next time the camera left the dude and returned, he has the moustache back.
Continuity mistake: When Dude is talking to the stranger at the bar the straw in his glass changes sides of the glass very often, but Dude isn't drinking in this scene.
Continuity mistake: When Donny bowls the last time, the time when he only hits nine, just as he bowls a man with a blue bag walks behind him, then it cuts to the skittles and then back to him, and the man keeps walking from where we last saw him as if he paused for the few seconds while the skittles were showing.
Continuity mistake: In the bowling alley, when Walter says "Am I wrong, am I wrong", there is a shoe on his left, then camera switches to the Dude, and when it comes back to Walter the shoe disappears.
Continuity mistake: In the opening scene when The Dude's head is repeatedly shoved in the toilet, almost all of the water is pushed out of the bowl and it's nearly empty. Soon the bowl is once again full, even though the toilet had not been flushed, which is the only way the bowl is normally refilled.