I'm imagining a person who found a spell that said it would make him immortal but instead it made him Undead without him knowing it, and rigor mortis got him outside. Flies and the like laid eggs on him for hours before he could move again. How long would it be before his sight went and his mind rotted in his skull? (Until that happens, he is an intelligent undead rather then a mere zombie.) How long could he pass as a living human for?