My favorite is the "Lemma that is not Burnside's". Also known as the orbit-counting theorem, the Pólya-Burnside lemma, the Cauchy-Frobenius lemma, and of course

My favorite is the "Lemma that is not Burnside's". Also known as the orbit-counting theorem, the Pólya-Burnside lemma, the Cauchy-Frobenius lemma, and of course Burnside's lemma.