I've locked up the rear twice. Once while stopping on a crosswalk in a short stop and almost hit the guy in front of me. And the once going down a nice hill with a hard right I was going in too hot and grabbed the brakes and as the road an imperfection the rear caught a little air/unsettled and I felt it slide out from underneath me, luckily I was already letting go of the brake and it just scared the **** out of me (the second instance was when I switched the crap textros to Ultegra on the maiden voyage of usuable brakes). I almost always grab the fronts when stopping and even when checking the speed in a pace line.