I have had a few encounters with Rendermen before, but like you, I though it was a rendering error. Here are some photos.
He's hard to see, but he's on the shelf, he has the emitter that happens after you F8/F7, the bubbles around him.
He's in the process of teleporting, I didn't even know I had this until today. Pretty blurry, but there was nobody on my server at the time.
You can BARELY see a face in this one, but its an ASCII terror face, I assure you.
Here is the same picture, but with a circle around the Renderman's head. (I edited for the circle and the more times you save a .jpg picture, the lower quality it gets. Look it up, it's true.)
Who is this mysterious figure!?