Fixing my broken, out of waranty, Canon EFS 17-85 lens.
There is a ribbon cable inside the lens that flexes as the zoom changes. After enough "zooming" the ribon cable breaks and the camera reports a completely unhelpful "Error 99". Three years after purchasing the lens, I had got the first "Error 99" and had it repaired by Canon. Three years later, here I am again. This time I decided to repair it myselft to save the repair cost Canon charges.
The suspect ribbon cable is the innermost part of the lens, so everything needs to come apart to get to it. I watched the youtube video, took it apart, replaced the broken cable, and put it all back together. In another 3 years I expect it to break again.