If you accidentally write on a whiteboard with a permanent marker, never fear! Re-write over your writing with a dry erase marker, then use a dry paper towel to wipe the marker away. The fresh dry erase marker breaks down the permanent marker and allows it to be wiped away.
Bonus tip: If you accidentally write on a different surface with a permanent marker (refrigerator door, mirror, etc) you can use hairspray, nail polish remover, or rubbing alcohol to remove it. Don't use these chemicals on plastics though -- they will eat right through the finish.
