When the film numbers show through onto the developed film roll it is called ' wrapper offset '
This is more common than people imagine.
Fundamentally its down to the quality of the film, its sensitivity (speed) and the quality of the wrapper used ( high quality 120 Film wrapper is a very special and expensive product ) It can happen with just about any film, yes, it can be caused by loading in bright light, something none of us ever do of course! and red window camera's are more prone obviously, but it can happen in any camera.
To lessen the risk, especially with red window camera's, always try and avoid loading a camera days or weeks before exposing the film, always use the window cover ( if it has one ) avoid elevated temperatures, and store the loaded camera somewhere dry, cool and dark. Once the film is exposed, remove it immediately and process promptly, if it is not possible to process promptly, remove teh film from the camera and store the exposed film somewhere dry, dark and cool.
Finally......many have commented ( complained ) on the lightness of the printing of the frame exposure numbers on ILFORD 120 film, this reduced print density helps significantly lessen the risk of wrapper offset on our products....by reducing the contrast between printed and unprinted areas on the wrapper it means less chance of the offset 'print out' on the film itself.
Simon. ILFORD Photo / HARMAN technology Limited :