When Mork is writing the letter to Superman, the paper changes in between cuts. At first, the holes are on Mork's left, then half way through the letter the holes change to Mork's right.
After Mork gives Mr. Bikley the puppy and he tries to get the puppy to sing, the camera quickly cuts to Mork and Mindy. In this shot, Robin Williams quickly breaks character and laughs as himself instead of Mork.