my friends dad has a brand new harley one day while riding he hit the back brake to slow down for a corner as he entered the corner and released the brake the back tire slid out, chuck fell off the bike whacked his head with no helmut on and now has mild brain damage all at around 15-20mph.

turned out the valve that lets the brake fluid go back to release pressure on the rear disc got stuck and the back brake locked the tire up right as he was leaning into the corner, so yes this stuff does happen.