Revealing mistake: In the dance sequence, we see Elaine twirling Ted around and throwing him into the crowd. Watch the top-right-corner of your screen, as the stunt-double lands in the audience you can see Robert Hays (Ted Striker) in the top-right-corner waiting to pop out. (00:19:10)
Revealing mistake: Right after one of the Mountie's cars is split in half by the snowplow, you can see the mini-wheel used to keep the half-car propped up.
Continuity mistake: Just before the Mystery Girl blows up the building in which Elwood resides, a police car with lights on its roof pulls up outside. When the building actually explodes just over a minute later, you see that the police car parked outside no longer has any lights. (00:39:54)
Visible crew/equipment: After Dudley's mom drops him off when he's wearing his magician costume, just as the girls call him over we can see four chalk T-marks on the ground in the wideshot, before Newbomb borrows his costume.
Visible crew/equipment: When one of them is using the bathroom - out of the bathroom window and it sprinkles on his neighbor's flowers, watch when the camera view goes up because you can see someone squeezing a rag onto the porch banister to look like his pee.