I also couldn't remember which special features segment it was, but in one of the DVD extras on a Lost DVD set - there's some random crewguy wearing a Pleasure to Kill shirt in the background.
And there's also a video that was on Tosh.o once, which includes a guy wearing a Blind Guardian shirt.
What are some other moments of metal shirts getting snuck into shows?
post by Quiet Observer at Jul 4,2014 5:11pm
...not quite on point, but in the British coming-of-age television show available on Netflix, The Inbetweeners, there is a Mayhem poster frequently spotted on the wall of the high school common room to the left of an NWA poster. I was pleased.