Stupidity: The Foot Clan knows that April O'Neil is an ally of the Teenage Mutant Ninja Turtles. Since the Foot Clan hasn't been able to find the Teenage Mutant Ninja Turtles for where they live a easy solution would be for them to go to April and ask where are the Teenage Mutant Ninja Turtles. An explanation is never given in the movie for why the Foot Clan never use that easy solution.
Suggested correction: First of all, the Foot Clan at the beginning of the movie is disorganized. Secondly, upon Shredder's return, he plants a Foot soldier named Freddy into April's news crew in order to keep an eye on her and find out where the Turtles are. Then they change tactics when they discover there is still some of the mutagenic ooze around, which they then decide to use to create their own mutants to lure the Turtles out of hiding.
Stupidity: Just after Ricky has beaten up Tarzan, the warden has the ceiling of the cell Ricky is trapped in lowered in an attempt to crush him. However, earlier in the scene, Tarzan made a huge hole in the wall of the cell to get into it. The hole in the wall is even visible in the background when the ceiling begins to lower. Ricky could have simply run out of the hole in the wall to get out, rather than try to hold the ceiling up with his bare hands. (01:02:48)