When R. (one of my private, 4th-grade students) wants to give you "joy dots," marker dotted on your hand so that you have a good day, and she wants to give you LOTS of dots, don't choose red. Otherwise people will think you have a disease.
What not to say to a roomful of African-American students, even if you're just talking about markers: "The black ones smell, the white ones don't."
What not to say to your teacher:
"Those red dots match the ones on your face."
After she's ended a horrific class with the previous group, in terms of behavior (theirs, not mine) (I think), even when she admitted she'd been acting like the bad witch: "You're acting like the good witch now." "But I'm still a witch?" "Yeah."
How not to conduct a timed assessment: Start the timer and say "go" without giving the student the directions. "WAIT! WHAT ARE WE DOING?!"