Revealing mistake: After Superman escapes the block of ice he was imprisoned in, watch the next shot (as he flies back towards Earth) closely... in this shot, it's clear that the effect was achieved by using a rather cheap-looking Superman "doll" composited into the shot, and not Christopher Reeve. It practically looks like a Superman "action-figure" you could buy at the store.
Revealing mistake: When Lex cuts the strand of hair, the bottom of the cabinet breaks before the ball drops.
Revealing mistake: When Clark and Lois are talking in Lacey's apartment, there appear to be bubbles and waves in the pool. However, they do not move.