Revealing mistake: When the party is falling in the gate found in the bedroom, they can be seen bouncing on what seems to be a mattress in the shadow. Yet, in the following shot, they are still falling. (00:10:55)
Revealing mistake: When Kevin and the Time Bandits push the wall all the way out, support wires can be seen attached to the wall as it disappears from view. (00:11:00)
Revealing mistake: When the party is falling from the gate for the first time, their shadows are visible on the sky. (00:11:10)
Revealing mistake: Right after the party splits in the fortress of darkness and Kevin and his piggy friend are chased by the big horned creatures, one of them fires a ball of flame out of its eye. In the next shot when they run trough the pillars of stone, you can see wires on the ceiling, the burning ball is draged with through the set (in the white smoke). This is even better visible some seconds later when the fireball flies trough the door. (01:30:15)
Revealing mistake: When Wally is firing the laser at random from the small spaceship, we see Fidget running down some stone stairs, and there are violent laser impacts directly behind him that could only have hit where they did if they had passed right though his body. (01:41:55)
Revealing mistake: When Kevin walks towards the ashes of his parents after the toaster oven explosion, smoke can be seen going back towards the ashes, revealing that it is reversed footage.
Answer: Probably diarrhoea, and he needs fruit for fibre. The "special..." he says he doesn't need to wear anymore might be diapers.