Other mistake: When Superman and Lois are flying through the clouds, there are a number of shots in which we see Superman from Lois' point of view. If you look closely, you can see that Christopher Reeve is not being suspended in these shots, but is, in fact, kneeling to look as though he is flying. You can tell by the profile of the red trunks.
Other mistake: Fred MacMurray is listed as "Clarence" in the closing credits, but when he is standing outside his store and talking to Maureen, a promotional sign in the background spells his name as "Clarance."