The true part of the story is that I really did tell the co-worker having a root canal done today about it being a "special day" for it, and then told her why.
But in this case, I was safe because
1. She wasn't in a terrible amount of pain, this was scheduled a few weeks ago.
2. She has a good sense of humor and took it in the spirit I intended, and gave an appropriate eye roll. She did wonder if the dentist would give a special discount for it, however.
3. I'm her supervisor and sign her evaluation sheets.
Of course, that has not always prevented her from threats of bodily harm in the past.....