This surprised the heck out of me. We recently finished a new TV room down in the basement. We have a 50" plasma TV, mounted on the wall with an Omnimount UCL mount (quite an impressive bit of engineering -- not cheap, but highly recommended.) Since the mount weighs upwards of 40 pounds, and supports a TV that weights 100 pounds on a torque arm as long as 2ft, it needs to be anchored pretty solidly to the wall. It is held up with 6 4" long, 3/8" wide lag screws, that screw into two separate studs. It does its job well.
So, my dad and I went to remove it from the wall. I removed the six screws, and we prepared to catch the mount. It didn't fall off the wall. We tugged on it, and it still didn't come off the wall. Seemed stuck so tight we thought we'd missed a screw! But we convinced ourselves there were no more screws, and the two of us pulled hard, and eventually it came off the wall -- taking some of the wallboard with it. Apparently the pressure of being screwed up against the wallboard (and maybe the heat from the TV too over a few years) turned the painted surface into a glue not only strong enough to hold a 40lb mount to a vertical wall but resist being pulled off!