The music building at my college has about 10-12 practice rooms (maybe a few more), all with a full length mirror and many with a piano. There's a couple in the front that have lights that resonate really badly with the D string. The first time I played in that room I would stop and inspect my violin every time I played a D loudly enough to get them to buzz. I thought it was coming from my violin at first.

I'd be pretty impressed with myself if I could that. B)

My teacher did tell me a story about one of his students throwing a chair out one of the (closed) windows.

I think breaking a window with a chair might be a lot easier than breaking a mirror with a violin though...