Revealing mistake: When they have to capture the crocodile, you can see that the mouth of the crocodile is tied or taped shut. When the crocodile turns its head to the side, the tape is visible. (00:32:55 - 00:33:20)
Revealing mistake: When Maturette dies at the end he cannot close his left eye all the way because of the totally noticeable fake contact lens he is wearing.
Revealing mistake: There is a scene where a prisoner is executed by guillotine. Just as the blade would hit the man's neck, the view/scene changes slightly. The dummy used for the man's head can be seen.
Answer: It does appear to be unintentional (or at least, unscripted), but they continue because when you're shooting a movie, you don't stop until you hear "cut", and especially at that time animal welfare wasn't necessarily a priority. I'm guessing no-one was concerned about the chicken, and so didn't feel the need to do anything about it. It's possible the film was made without an animal welfare monitor on set. As to why it's in the movie, the whole "marching to the prison" sequence was probably handled by the assistant director (as shots like this, not involving the principals or any substantive dialogue, often are) and they may have only done the one take. Who knows, they may have thought the injured chicken added realism to the scene.