In my second year of teaching I was tired, cranky, and in the middle of reading the worst student essay I had seen at the time. Leaving that school soon (due to graduation) I had little to lose. In a plagiarized paragraph about space exploration in an essay NOT about space exploration I wrote "This drove me to chocolate."
In my head, I formulated a plan--if student writing drove me to pull out the Hershey's they would be told. Surprisingly enough, this has never shown up on my teaching evaluations or even on RMP. Today, I simply write "Chocolate" next to passages that drive me to indulgence.
However, over time I've ran into worse problems. These aren't shared with the students (as much as I'd like to), but if I did write on their papers some passages would be labeled with "Beer."
Today I added a new one. Dear student who cheated on a resume and was caught by failing to change the name across the top in 20 point font: For you, I reserve the label "Absinthe."