This isn't movie mistake, but it is a famous piece of soap opera legend. Back in the early seventies (I think), on the soap opera All My Children, the oldest son of the characters Jack and Ruth Martin went upstairs to his room to wax his skis and was never seen again.
Wasn't it on that same show that one character discovered that he had been sterile all his life. Which got him to wondering why his son bore a family resemblance. Only to discover that his brother had been bopping his wife.