Revealing mistake: About 1/3 through the episode, they are in Roz's apartment. If you watch when people enter or leave the apartment, the door has a handle lockset. It also has a lock cylinder about 1 1/2 feet above it on both sides of the door. The problem is, the side of the door has no drill hole where the latch assembly should be: it's just solid wood. The lock isn't attached to anything that would enable it to work.
Revealing 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: When the power outage occurs, Daphne claims she can't cook the steaks, however, the cook top in the kitchen is gas. They have matches as shown by them lighting the hurricane lamps.
Factual error: There are many factors concerning Niles' post-operative state which are very inaccurate. After open heart surgery it takes at least three months for the chest bone to heal, Niles would not be able to drive a car for weeks, nor would he be able to lift or twist which he does on the show. Also, he doesn't sport a scar. Your chest bone begins very high up, just below your neck, his scar should have been visible on numerous occasions.