"Well," said Albus Dumbledore. "I do feel stupid."

"I should hope so," Professor Quirrell said easily; if he had been at all shocked himself at being caught, it did not show. A casual wave of his hand changed his robes back to a Professor's clothing.

Dumbledore's grimness had returned and redoubled. "There I am, searching so hard for Voldemort's shade, never noticing that the Defense Professor of Hogwarts is a sickly, half-dead victim possessed by a spirit far more powerful than himself. I would call it senility, if so many others had not missed it as well."

This is Dumbledore admitting he held the Idiot Ball. We've been promised nobody's holding the Idiot Ball. So something's up. I like the theory that mirror is operating as intended and showing Voldemort what he wants to see, that being Dumbledore making a mistake and losing. Alternately, it could be that Dumbledore was somehow able to make a version of him inside the mirror (maybe in his CEV he saw himself inside the mirror protecting the Stone, and that reflection-Dumbledore gained independent existence?). Or he could just have something else up his sleeve.