In the episode where Cory makes a tape of Sean, Jack, and Eric living in their new apartment, after showing the tape in class, Mr. Feeney hands it back and Cory places it on his desk. When the class is over, he leaves the tape on the desk and never goes back to get it.
Why would Mr. Feeny have given Cory the tape back before he graded it? That doesn't make any sense whatsoever. That would have put him in the position to have to go back to Cory's desk after class and retrieve the tape. I think this is a genuine slip-up.