Continuity mistake: As the two look-outs get on the big wheel the thin guy with the ventriloquist's dummy sits on the right. But later in the film he is on the left.
Continuity mistake: During the scene when Wally gets hit by the soldier in the cafe, and Wally bumps into another soldier, causing him to go into a cake, take notice of how much cake is on him throughout the different camera shots. Also Pops (the owner) starts to wipe the cake off of the soldier's face. The amount of cake changes between shots. (00:09:20 - 00:10:15)
Revealing mistake: When the Ferris Wheel at the theme park has broken from its mounting and is rolling down the pier, it appears to be rolling forward but rotating anti-clockwise when if it is rolling forward it should be rotating clockwise.
Plot hole: When Ward's son hits the switch marked "Do Not Touch This Switch", every light in the amusement park comes on, music starts playing, and the rides all start moving. It's a funny gag, but it seems unlikely that one switch would power every single thing in the amusement park, and that everything in the park has been left on "full speed." If that's the case, when Angelo first sent the two lookouts to the top of the ferris wheel, everything else in the park should have come on then too.
Continuity mistake: When the Army knocks a hole into the front door of the Douglas home with the anti-aircraft gun barrel, notice that the position of the hole changes between shots. And when the soldiers pick the door off of Mrs. Douglas you do not see the hole. Also notice that the door is standing back up and intact, in the background, as the soldier explains to the Douglas family about the gun. (00:29:30)
Revealing mistake: The actor who played "Wally" did all of his own dancing during the big dance scene, except the part where he tap-dances down the long table of food. We see Wally jump up onto one end of the table, then the shot cuts to a close-up of his feet as he tap dances among the plates and glasses. When he reaches the end, he jumps up into the air and drops into frame, showing that it's really him. Since the actor couldn't tap dance, the film's choreographer did the tap dancing part. At the end of the table they hung a trapeze bar and the actor hung from it with his feet pulled up. When the choreographer reached the end of the table, he jumped up into the air and grabbed the bar just as the actor let go, so it looks like the same person. But if you look very carefully, you can see the actor's shoes dangling above the edge of the table just before the choreographer jumps.
Visible crew/equipment: When Wild Bill crash lands his P-40 Tomahawk in the street, then dazedly tries to "bail out", watch his parachute. A nearby explosion is supposed to catch the parachute and pull him backwards, but the 'chute doesn't react to the explosion. Instead, a line attached to the end of it pulls Wild Bill backwards.
Revealing mistake: During the riot scene, "Stretch" drags Betty under a truck so he can rape her. Wally drives the tank across the street and bumps the truck, knocking it over (so he can save Betty). If you watch, the tank bumps the truck, there's a pause, and then the truck flips over. It is obviously pulled over by something off-screen and not by the tank.