Continuity mistake: When the two different brothers retrieve the book from the Narrator (and ultimately shoot or get shot), in almost every shot, you can tell the scenes were shot separately by looking at the background. Donald Sutherland's shots are different by the wind hitting the plants in the background; Gene Wilder's shots are different by the position of the sunlight (i.e. one is dark, the other is fully lighted). Obvious if you look closely.
Continuity mistake: If you follow carefully, the twins were not actually mismatched at birth.