At the start of Community S01E04, Professor Chang starts the class off by thanking people for their feedback on his teaching, before saying:
"But there was one evaluation that was so harsh, and hurtful, and racist..."
At this point, Pierce begins to hide his face before it is revealed that the evaluation Chang is talking about was written by Annie, to which Pierce seems surprised.
What is the joke here? I originally thought that Chang had matched up the handwriting wrong, but seeing as nothing came of it I suppose it's just because Pierce seems more likely to write hurtful and racist things, and he (and the audience) knows it.