Comment by aidenn0

A friend of mine was given an assignment in a masters-level CS class, which was to prove a lemma in some seminal paper (It was one of those "Proof follows a similar form to Lemma X" points).

This had been assigned many times previously. When my friend disproved the lemma, he asked the professor what he had done wrong. Turns out the lemma was in fact false, despite dozens of grad students having turned in "proofs" of the lemma already. The paper itself still stood, as a weaker form of the lemma was sufficient for its findings, but still very interesting.