When Richie falls off the ladder, it's apparent from the way he falls that he lands on his front (you just see his leg pop into shot) and that his head would be pointing towards the camera. However, instantly in the next shot we see him on his back with his head towards the wall.
When Richie is telling the other three guys to go to the corner, he still has his hands on the wheels of his chair. But when the shot changes and Richie says, "If you need the toilet then put your hand up", his arms are crossed around his waist.
When Eddie ties the string to Richie's leg and slams the door, the jeans actually rip and the fake leg comes out and you can even see the actor's real leg tied underneath.
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.
When Richie falls and breaks his leg, the board which has the fake leg on is visible.
When Richie and Eddie are trying to put Richie's leg back to the right direction, the clicking sound of bones breaking carries on even when neither of them are touching the leg.
When Richie is pulled towards the door by his broken leg, the wheeled trolley he is laying on his visible, along with Rik Mayall's real leg.