The lens is bein shocks or pressure from the front against the lens is very sensitive, it can lead to failure of the focusing drive! Here already is sufficient, when the lens is mounted under slight pressure in the camera bag. This problem occurs frequently, as shown by the relevant Internet forums. The repair cost me 120 euros!
The solution: Lens hood Canon ES-71 50mm 1 II Lens Hood for the Canon EF: order the 1.4 USM lens and always leave it. Since the lens hood is not fixed in this lens on the tube, but the housing, it keeps the shocks from the tube and improves the way considerably the contrast and color brilliance.