When you accidentally write on your whiteboard with a sharpie...

1. scribble dry erase marker over the top of it
2. wait thirteen minutes (or so)
3. erase!

I just found this trick by Googling and ending up at http://www.wikihow.com/Remove-Permanent-Marker-from-a-White-Board and I endorse this approach. By the way, this is also the way to get rid of really old, stubborn, dry erase marker scribblings which have been left to sit for too long.