Revealing mistake: When Ade Edmondson clamps the pliers onto Rik Mayall's nose, if you watch carefully you can see the pliers bend a couple of times, showing that they are made of rubber or something similar. (00:13:45)
Apocalypse - S1-E4
Revealing mistake: After the piano falls from the ceiling, Ritchie rumbles Eddie's "plan" and kicks him out of the flat. When Eddie is pushed out the front door and down the stairs you can see him falling to begin with, but halfway through he actually stops as if he's kneeling, then ducks out of shot. The sound effects lead us to believe that he's fallen all the way down the stairs and been attacked by a pack of dogs at the bottom.
Apocalypse - S1-E4
Revealing mistake: When Eddie falls through the ceiling playing the piano, it's very obvious from the back that it's a dummy because it wobbles around but is stuck to the seat.
Revealing mistake: When Eddie ties the string to Richie's leg and slams the door, watch the leg. The jeans actually rip and the fake leg comes out and you can even see the actor's real leg tied underneath. It's really obvious in slow motion.
Revealing mistake: When they are leading Richie to the cupboard while playing postman's knock, Dave Hedgehog puts his hands on the wall. The whole stage wall shakes when he touches it.
Revealing mistake: During a few scenes after Richie's finger has been taped back on at an extreme angle, you can see Rick Mayall's real finger bent underneath it. This is most visible after his finger gets slammed in the window: look carefully after the window is opened.
Revealing mistake: Even though you hear it ticking, the second hand on the clock in Eddies' room doesn't move.
Revealing mistake: When Richie says "Don't you dare call me overweight young man" and tips the ferris wheel cart over, you can see that it is a dummy representing Richie.
Revealing mistake: During a few of the "flaming fart" scenes, you can see a wire connected to the bottom of the actors. It's hard to see because it appears that it was attached to their legs in some way, but you can still see it when they move around a lot. The easiest place to see it is when the "devil" knocks on the door and they all fart and jump around, look carefully between/to the side of eddies legs for a black wire. It's not the dressing gown belt because that's tied around their waists.