First semester teaching, about 45–50 college sophomores and juniors. We were covering matching and search-and-replace in Perl. In Perl, the operators for matching and search-and-replace are m// and s/// respectively. At one point, in summary of the lecture I had just given, I said to the class, “Okay, so remember, it’s S & M, folks, S & M”. The class burst out laughing, my face went beat red, and I vowed to remember to always refer to them in the opposite order from now on.