Revealing mistake: When Jack's wheelchair is at the top of the cliff and about to fall off, it's obviously a stuntman and not Frank Kelly sitting in it. Similarly, when the wheelchair falls, the dummy used isn't even dressed like Jack. (00:14:25)
Revealing mistake: When Ted and Dougal are climbing the big rock after seeing St. Kevin's Stump, it's so obvious that a light is being shone on them from behind. They both have shadows even though it's a dark and cloudy day.
Revealing mistake: At the end when Ted flushes the toilet and kills the priest, we see an outside view of the priest and if you look at the bottom of his robes you can see a black square thing hanging down from the inside which must have been squirting the water out of his robes. Also to prove this, you are not able to see any water coming from the drain in the ground, which does not make any sense.
Cigarettes and Alcohol and Rollerblading - S2-E8
Revealing mistake: When Ted jumps onto his bed (when the nun replaced their mattresses with bricks), you can see both beds are bouncy when Ted lands on his bed and Dougal lifts back the covers on the bed half way through the episode. (00:19:20)
Answer: Ted was using his real hands to control Father Jack's wheelchair using a remote control - the fake arms were so no-one would see him cheating.
Sierra1