“DNE” written on a whiteboard or chalkboard is supposed to mean “do not erase”. When I was a student at the University of Georgia, I saw it only infrequently, but it seems to have been more common elsewhere, or in any event, become so subsequently.
Randall Munroe, of XKCD fame, called attention to “DNE” usage with this strip several years ago:
Recently, I decided to test for myself how sacrosanct something marked with “DNE” would actually be in practice:
When I added that, the board was full of random gibberish from various meetings that had been held in the room. Now it’s blank — except for my circled mystery code.
Two weeks and counting.