I had a chimney fire and my flue was just swept yesterday!
Thankfully, this does not occur frequently, but when it does occur this does not mean that the sweep did not do his job well. Taking digital photos of the flue system, every mortar joint, every flue tile, will help immensely when these problems arise. Often times after sweeping, all the chimney sweep can see is a thin, smooth glazing of creosote in the flue system. Such a thin layer does not seem to warrant rotary cleaning, nor does it allow the sweep to see exactly what is going on with the flue tiles themselves.
Creosote will then curl up into little flakes, making a perfect environment for a chimney fire to occur. If the flue system reaches a certain temperature, the creosote can combust and cause a roaring chimney fire.
Because such a situation is possible, it is absolutely crucial for a sweep to document his work in the home. If you have a sweep come into your home who has no documentation or cameras for pictures, it would be wise to promptly send him home. Remember, you will need pictures of your flue system to get money from your insurance for any repairs of a natural occurrence and you will need documentation of the sweep and procedures done for your own peace of mind.