Revealing mistake: When Albert and Harold are fighting over the cracker Albert's face goes into the trifle, and if you look carefully we can see that he just lets go of the cracker and puts his head in the trifle deliberately.
Factual error: In the opening scene, the Clampetts are driving home from Malibu. From Malibu to Beverly Hills is east. But as they head home, the ocean is on their left, which means they are driving west.
Continuity mistake: Halfway through the show, they get caught not having the boat, and McHale gets taken by boat to the commander's office. As he leaves the pier, he drops the rope on the seat next to him, they do a close-up and the rope is behind him about 5 feet away.
Audio problem: When Judy asks "But Grandpa, why didn't you tell us what was going on?" towards the end of the episode, she is speaking with Jane's voice.
Revealing mistake: The boat is quite rocky, causing their food to move around the table. However, the plate obviously has wheels on it to make it move easier. (00:18:00)