I guess I am a bad man, because the Medusa my players fought was a vampire. Oh boy the glares I got when they got ambushed by the villain who was standing right in front of them was priceless. She had no reflection of course and they were relying entirely on mirrors.
One had even made up a nifty little periscope to try and see her with. He went to great pains to describe how hard it was to take off his periscope helmet, thinking to thwart any DM bastardry on my part. Seeing him spend 2 rounds taking it off, only to get turned to stone once he did still warms my evil heart.
Of course, I did not tell them that the villian was a vampire, it took them a while to work that out. Once they did though, the clerics liberal use of AE radiant damage and turn undead turned the tide in their favor. It was a close call though.