Revealing mistake: When Jonathan is falling down the stairs when being chased by the ape, there is 1 shot where you can see it's a stunt man. Slow mo may help but is not essential.
Continuity mistake: The way the pumpkin is positioned when Maddy is trying to cut it keeps changing depending on the angle. It is either on its side or upright.
Continuity mistake: When Jonathan falls down the stairs when being chased by the ape, his trousers go from black, to light blue, to black again.
Chosen answer: It was a bottle of turpentine.
Methylated spirit, always coloured purple.