When the Turtles are eating pizza at their table and talking to April, Donatello's head almost pops off as he swallows his piece of pizza.
When Donatello leaps in font of April, he speaks in Raphael's voice the first scene, and when it cuts back to him, he speaks in his own voice.
The mutagenic gel is supposed to mutate whatever it affects into something resembling the last creature that it touched, yet Splinter had last touched the Turtles, not a rat.
If the mutagen turns you into what you last came into contact with, then how did Splinter completely become a rat and the turtles only became bigger turtles? Should they have become fully people?