Revealing mistake: In the part in the motel where THING is doing morse code on the table to Gomez you can see the cut-out in the table where the guy's hand is sticking through the table.
Revealing mistake: When Rufio brings the sword down on the rope holding the slingshot, you can hear the intended sound effect, but the rope is not cut. (01:05:20)
Revealing mistake: In the junkyard, Leo and Raph grab a long pole and plough through several Foot Ninjas with it. It is painfully obvious the ninjas react to the pole before it ever hits them. Especially the guy on the ground. (00:53:20)
Revealing mistake: When Ernest's jack flies out from under his truck it breaks the back windshield of the mayor's car. Watch closely and you can see the glass shatter before the jack makes contact with it.
Revealing mistake: When the king's youngest daughter mounts the Polar Bear King the lack of effort reveals that she does this with the help of a cunningly placed stepladder. (00:25:35)
Revealing mistake: Junior can't get on the Crazy Dance because he is a milimeter too short. After seeing Murph (the 20-year old in his class) and Trixie (on heels) get on it, Junior speeds up the ride and all the kids on it get dizzy and barf everywhere. If you look where the puke shoots out, it's a bit obvious it's not coming from their mouths, but from a point behind them. Most obvious are Trixie and the boy wearing glasses.
Revealing mistake: In the first shot of the club scene, where we see the clarinet up close, the ligature (the metal band that holds the reed in place) has been placed upside down. This would make it extremely difficult, if not, impossible for the clarinet to play a single note. So, the "clarinet player" physically could not play his instrument, even if he wanted to. (00:51:00)