Revealing mistake: When Bluto is watching the sorority girls have their pillow fight and he falls down with the ladder, you can see him land on a grass mat that is cut into the lawn. It's funny because you can see the grass push in. (00:40:00)
Visible crew/equipment: In the scene during the American bandstand, at the end of the song when the teacher is dancing in front of the band members, the crews can be seen. The dressing and the hair doesn't match Grease era at all, and someone is sitting on a director's chair as well. (01:06:20)
Visible crew/equipment: Near the beginning when Lois and Clark are walking out of the Daily Planet, Clark gets stuck in the revolving doors. But, look closely at the glass door, and you can see Richard Donner (confirmed in Superman expanded commentary) holding the door so that Christopher Reeve can get stuck.
Visible crew/equipment: After Matthew and Elizabeth take the pills to stay awake, when they are found Matthew shouts, "David, don't let them touch her," and there's a visible T-mark taped to the floor in the overhead shot. (01:31:30)
Continuity mistake: During the first meeting between Lou and Madeline Kahn, her fur stole changes position between shots almost more often than she changes names. (00:09:30 - 00:10:30)