When Pierce sits down with Troy to discuss his image and sneezes, he brings two unopened soda cans. In a shot later we see the can opened and partially crushed without being consumed.
When Abed is filming Jeff and Britta for the final scenes in his movie, Britta screams "Why won't you answer me!?". During the actual showing of the movie, Britta screams "Why will you not answer me!?".
When Professor Whitman gives Britta a flower, he has his hand on his chest. When the same scene is seen from another angle, however, Professor Whitman's arm is parallel with his body.
When Pierce is teaching Troy how to sneeze around 11 min mark, both cans of pop look normal. At the end of the scene one of them is crushed for no reason. The scene as shot was much longer, and it was cut for the episode.
When Pierce is talking to Troy about his array of sneezes, any time the camera is facing Troy, Pierce's mouth does not match up with his lines.
Around 8 min 30 sec marking tape is visible on the ground that marks where each character has to stand during the scene.
In the movie Abed made, there was a subtitle "I think the wrong person just left" it was wrongly translated into Arabic (most likely by mechanical translation), mistranslating "left" as a direction. An Arab would not make this kind of language mistake.