We let Anthony use some markers the other day. We supervised him for a bit but then had other things to do.
The kid was alone with the markers for only 15 or 20 minutes. He ended up with ink all over his forehead, hands and fingers and all over his shirt. When we realized the extent of the damage to him, we started looking around the house and, lo and behold, there were hand prints on the wall going up the stairs!