Working dry erase markers. You don’t realize until you need to whiteboard something and all the markers are dried out and useless.
He'd been drawing an elaborate picture on our white board with a permanent marker some idiot transferred from a paper easel. He got nearly the whole thing drawn without any mistakes, and only as he was finishing did he realize he couldn't erase it. Boy was that an emotional roller coaster.
I've watched I don't know how many people cycle through three dead markers on a white board and put them all back. The only way a conference room ends up with 3 dead markers is because it has 3 dead markers and nobody does anything about it. When there are no markers left new ones will show up, one way or another.