There have been numerous topics around here about this Canon lens failing after a slight bang on the noggin. Many opinions have been posted, but the main idea that everyone seems to agree on is that the USM's FTM "clutch" tends to break. Well, mine just happened to get hit by this problem on Monday, and, since I don't have any warranty anymore, I decided to take a screwdriver and take it apart. My conclusions were mixed (i.e. I'm very disappointed by the build quality of the inner parts of the lens), but the main idea is that it's not the USM coupling that breaks. Actually, nothing really breaks, but rather, the focusing helicoid, which is made of really cheap plastic, bends out of shape by maybe half a millimeter, and blocks against the main assembly. For me it was a very easy 15 minute fix, and now it works perfectly. If anyone is curious I can do a short tutorial with pictures and all, if you want to attempt a DIY job. Please be warned, UNSCREWING EVEN ONE SCREW ON THE LENS VOIDS YOUR WARRANTY!