Allan,
Imagine you have removed both ends of a metal cassette, and look down the hollow. The more recent ones have the velvet light trap positioned tangentially to the main cylinder, but for some earlier Agfa and Ilford (perhaps others) cassettes, the light trap is at an angle to the cylinder, where the film has to emerge out of the coil after negotiating a small reverse kink.
The British photography writer A.M. Carlsson first noticed the adverse effect of cassettes not allowing the film to emerge tangentially. After leaving the film in that kink for a while (overnight or longer perhaps), the film will develop a reverse bend that cannot be flattened out by the pressure plate. So you will see the following happening:
After you have started a film, you put your camera aside overnight, and then when you wind on again, the piece of film with the bend will be in the film gate, but since the film is not flat, you will see a vertical band where sharpness is lost. This is particularly obvious when you are photographing at infinity, where the distance between the lens and the film is actually less than what it takes to achieve infinity focus. But this can only happen during the first half of the film; as the coil of film gets smaller in diameter, the angle of the velvet light trap becomes progressively more tangential, therefore no longer putting that bend on the film.
For me I still use those older cassettes, but since I shoot off a roll quite rapidly I do not have much of a chance to let the bend to set in anyway.