Go to the bathroom scene where Fletcher's beating himself up and wait until he smashes his head in the toilet and watch the white rim of the toilet. It will bend like rubber. Watch carefully.
Well think about it, Jim practicly beats the hell out of him self all threw the entire scene! Why would they just make the toilet fake? why not the whole thing?
This is not a slip up. If you look closely you will see that the toilet seat is one of those that has no connection in the front of it. therefore it would have a "rubbery" look to it while he was bashing it on his head. Don't believe me next time you see a toilet seat like that try it on your foot.
Of course they didn't want Jim to hurt himself, so some parts of a set-up will be built for safety. The slip-up in this case is that when the toilet is SHOWN bending beyond a realistic amount, that it was not spotted and cut.