Revealing mistake: When Mork rollerskates into the wall, the wall moves slightly.
Revealing mistake: Before the officer comes in, Mork is 'saving' eggs. When Mork puts an egg under the cushion of the first bar stool, the cushion's underside is already wet or sticky with what looks like egg.
Snowflakes Keep Dancing On My Head (a/k/a Sky Flakes Keep Falling On My Head) - S1-E17
Revealing mistake: There are several places where Mindy does not favor her "sprained" ankle. The most noticeable is when she and Mork are about to open the trunk, she shifts her weight from one foot the other a few times.
There's a New Mork in Town - S3-E13
Revealing mistake: During the sword fighting scene, the split in the couch is visible before it actually gets split. Also in this scene, when Xerko has Mork at sword point right before Bickley comes in, Mork's ribbon is cut in the close up, but in the long shot, it's intact. Xerko will later cut the ribbon.