Wow, it's been forever since I've posted a brain-melty optical illusion, and I love me some illusions. This one is a variation on one I've posted before, showing how an ambiguously lit object freaks your brain out trying to figure out the lighting.
I really wish we had a show as smart and wonderful as QI here in the US. I love Stephen Fry as much as I do illusions!
Anyway, there are lots of examples of this type of illusion. Here's one of a silhouetted figure spinning that'll destroy you. This is also similar to the dome/crater illusion as well, where something (a crater) lit from below makes it look like an inverted object (a dome) lit from above.
I have more illusions listed in Related Posts below. Seriously, click the Blue/Green link. Oh my, that one is AWESOME. And people still argue over it in the comments, despite my complete and rock solid proof of what you're seeing in it. Optical illusions really can mess you up.