Continuity mistake: When Benjamin is running up the stairs of his rooming house with a stack of presents and a big bouquet of flowers, the flowers are pointing down held by his left hand with the presents in front of him. When he rounds the corner of the stairs, the flowers are upright in front of both him and the presents.
Continuity mistake: Benjamin follows Elaine and meets up with her on a bus as she's going to the zoo to meet her boyfriend. After Elaine and her boyfriend walk off and leave Benjamin alone he looks over at two monkeys sitting in their cage. The first time we see them they're sitting next to a window, but in the next shot of them we're looking at two different monkeys that are not sitting next to a window.
Continuity mistake: Benjamin uses the cross that he pulls off the window to keep the wedding guests away from Elaine and himself. Right before they exit the church he charges the bystanders one last time then turns around and holds the cross as if he's going to swing it through the door. In the next shot he's just holding it outstretched in his hands.
Continuity mistake: Mrs. Robinson sits at the bar and asks Benjamin, "Did you know I was an alcoholic?" and the silver shaker bottle with the large ring on top of it moves from the back of the bar to being between Mrs. Robinson and the radio she turns on, on the left wall. A few shots later it moves back again.
Continuity mistake: Benjamin breaks into the Robinson's house and runs upstairs looking for Elaine but finds Mrs. Robinson instead. As she picks up the phone to call the police we see nothing on the bed except a blue pillow. In the next shot there's a lot of other things now covering the blue pillow.