A recent comment on a Reddit posting:
At my company, we had some guy from another come in to go over some things with the bosses and a few key people. After his presentation, they turned off the projector, and he whipped out a dry-erase marker and started writing notes on the white board for everyone while my bosses were frozen in shock. When one of them could finally speak again, he let the man know he was drawing on our rather expensive projection screen.
The same thread also had a solution to the problem:
You can erase “permanent” marker ink by first coloring over it with ink from a washable marker. The two are essentially the same, but the latter has an extra solvent that makes it possible to remove.