That D20 is pretty large, so I guess you could do some pretty bad damage with it.
The players in the Amber campaign were amused by the shirt (Amber is a diceless RPG, so it was like 'cursing in the church' as they say in the Netherlands).
When I was in highschool, I once broke the glasses of a friend of mine with a D20 -- a standard-sized one, not a monstrosity like the one I'm holding on the photo. We were playing an RPG (I think it must have been Rolemaster), and I was GM'ing (as usual). I'm sure the characters were engaged in some sort of social interaction, because B. became bored (he was a bit of a hack'n'slasher) and he started throwing his dice at me. He hit me in the nose with a D10.
So I took out a D20, and told him to open his mouth and close his eyes... He laughed and complied. I threw the D20 in his general direction (aiming for his mouth) and hit his glasses, which shattered on impact. Luckily, he didn't get any glass in his eye, but he had to get a new glass -- he had the special high-density glasses too...
This taught me that dice can be deadly weapons, and that D20s are to be handled with great care!