While not necessarily a mistake, Bob waking up in the original Chicago bedroom with Emily, despite mentioning they would move away to Oregon in the series finale, Happy Trails to You (1978). They must have changed their minds.
Right before Larry, Darryl and Darryl show up, four other former townsfolk show up and are talking to Dick. Right before the townsfolk are done talking to Dick, a boom mic can be seen dipping into the scene for a few minutes, before pulling back up and out of sight.