Poppy's By the Tree (2) - S2-E2
Revealing mistake: When Al is blocking the killer's axe with his suitcase and they struggle for it, it is obvious that the axe is made of rubber, as the handle keeps bending.
Poppy's By the Tree (2) - S2-E2
Plot hole: Al and Peg are upstairs with the killer. Both of them are screaming, but downstairs, you can't hear anything. Everyone is sitting around talking. It's not until Al and Peg scream again that everyone runs upstairs. And you can tell the screams were "on cue" also.
Answer: It was one of the hoses connected to the engine, nothing specific, but to indicate that Al tampered with the vehicle.