Saw this one caught on the news a few years back:

A would-be thief hauls a cinder block (a.k.a. Concrete Masonry Unit, or CMU) up to a storefront's plate glass window with the intention of breaking through the glass and stealing some goods. He heaves the cinder block at the glass only to have it bounce back (he failed to note that the storefront was made of plexiglass) and cream him square in the face.

Wish I could find that video somewhere.