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.
Audio problem: When Bishop Brennan calls for the first time at the start of the episode, Ted panics and pretends to be French before hanging up the phone. You can still hear Bishop Brennan talking on the other end of the line for a few moments after Ted hangs up, which obviously shouldn't be possible.
Continuity mistake: Ted and Dougal are sitting in the fourth row from the back for the journey. But when Ted presses the emergency button and after the credits, three completely different men are sitting in their seats. Everyone has been given seat numbers so they should not be sitting there.