Other mistake: When Sylvia is outside the Sheffield mansion she says "I wonder if my daughter Fran is at home? I will see if she is" the doorbell rings as soon as she finishes that line, but you can see both her hands are visible. She can't have rung the doorbell.
Other mistake: The guy who plays Clive in this episode has one of the most appalling accents I have heard. He's apparently from Manchester, but sounds like a sort of cockney South African. His accent slips around all the time; it's very poor. I wouldn't normally submit it, but his is so shockingly bad I doubt anyone would be fooled into thinking he was British.
Other mistake: Throughout the episode, including the very first shot, we see the sun is shining brighter and hotter than usual. This lends credibility to the theory that Superman's powers are responsible for increased solar activity. Later on in the episode it is revealed that the heatwave is being orchestrated by Lex Luthor, who is is heating an underground river to coincide with Superman using his powers. Lex does not however have any control of the weather, and no explanation is offered for why the weather conveniently 'played along' with his plan.
Other mistake: When Cory and Shawn are auditioning for "Guys" and they're talking about the first person they pick, Cory says,"He's shorter than me and..." but after they pick both guys and he's talking to them, you can see that Cory is the shorter one.
Other mistake: In the scene where Yakko, Wakko and Dot are sitting at their desks repeating Miss Flamiel's every word, Wakko is drawn without legs (only feet) during one shot.
Beavis and Butt-head vs. the Vending Machine - S4-E28
Other mistake: When Butt-head is purchasing the pork rinds from the vending machine, he notes that the selection for them is F2. When he is pressing the buttons to dispense them, he presses the F button twice, but never the 2 button. Somehow, the machine still dispenses the correct item.