Continuity mistake: when the kids are telling Beaver that he has to climb into the teacup, the kid with the heabands hair keeps changing. In one shot, it's hanging over the headband, and in the next the headband is in front over the hair, and it keeps going back and forth.
Continuity mistake: In the shots where Beaver and his dad are driving in their car, the car behind them keeps changing. It is really close, then really far away, and then really close again, faster than the car actually could.
Factual error: At the end of the movie, Wally pushes Kyle into the big vat of fudge after he made fun of the Beaver. When fudge is in liquid form, it is extraordinarily hot, and would have burned the child severely.