John's list is very thorough so I'll just comment on a couple of things. We've successfully found: plugs in liquor lines, sludge in liquor tanks, foam levels in soap tanks, bypassed heat exchangers, wet streaks in felts, dryer cans not syphoning correctly, lime kiln brick delaminating, mud rings in kilns, ClO2 short-circuiting inline mixers, partially plugged blow lines, channeling in stock towers, belts misaligned, conveyors dragging on troughing/return rolls and numerous starter faults (uneven phases, bad fuse clips, loose connections, verifying heater strip operation).
The key is knowing how to operate your camera and it's limitations. Just walk around with it for a day or so and look at your processes. It will pay for itself guaranteed.