You will notice that on external shots of Jerry's building there are insect screens on the windows. However, these are never evident whenever any of the cast occasionally lean out the window whilst the scene is being shot from inside Jerry's apartment.
In two episodes, the makeup Michael Richards is wearing comes off on another surface. In "The Pick" when he falls against the wall in Calvin Klein's office there is an obvious dark mark left on the wall. In "The Butter Shave" the makeup is clearly seen on the stick on butter after he smears it on his face.