"Transfiguration can be undone too," Richard cut in. "Even if you don't know what something was originally, you can return it to its natural state if you know that it [i]is[/i] transfigured. But that's the hard part, isn't it? A teapot with legs is obviously charmed, but most people wouldn't stop to think that an old tire is actually an alligator or something..." he trailed off, his eyes widening slightly and his usual thoughtful movements ceasing in a characteristic pose of having realized something. "Be a good way to hide a body," he finished quietly. 

"Erm, yes," Clowers looked at Richard with mild concern. "In fact, there are several documented cases of that happening. And of course most experts assume that it has happened many times without ever being discovered. Typically it's found because something is out of place. Or else someone familiar with the scene(a resident at a home, for instance) points out that some object has never been there before. And of course an inept or not completely trained wizard might make errors in the transfiguration."