Sweet, thanks all. I definitely didn't switch it on purpose. I took a picture of some Thai kids and showed them the result. One got all excited and started pressing buttons. Perhaps that's what happened.
Any who, after reading about it, it actually seems handy.
Do you guys use the mirror lock up feature often?? I enjoy night photography and it seems useful.
I agree about Mirror Lock up being useful for slow exposures on a tripod.
Liveview is a good way to go too as it locks up the mirror. Use Liveview ESPECIALLY if you are doing multiple exposures for say, HDR. Much faster. The mirror locks up just once and all the exposures are made, and then the mirror goes back down. Very convenient.
Vibrations for the mirror are significant. I was in an abandoned factory a couple of weeks back and was using a carbon fiber tripod, with weigh on it for added stability. I was shooting on a solid concrete floor. I could actually feel the vibrations from the mirror in the balls of my feet when I was doing my test shots. At first I thought I was mistaken...but I did some more tests and amazingly it was true. To me...that was significant vibrations from that mirror. Amazing.
5D III +BG, 5D IV +BG, Canon 15mm f/2.8 Fisheye, Rokinon 15mm f/2.4, Canon 17mm f/4L TS-E, Canon 16-35mm f/4L IS, Sigma 20mm f/1.4 Art, Canon 35mm f/1.4L II, Canon 24-70mm f/2.8 II, Sigma 50mm f/1.4 Art, Canon 85mm f/1.2L, Canon 100mm f/2.8L Macro, Canon 70-200mm f/2.8L IS II & 1.4x III