Quote Originally Posted by AgX View Post
That's why there are cameras that wind the exposed film into a covered chamber, or wind the film at the start completely onto an open spool and wind exposed film back into the cassette.

Strange enough there seems to be no design with a secured back door lock that only opens when there is no uncoverd film in the film chamber. (This needed to include an overide in case of malfunction).
Well, it's only a problem if you want it to be.
When you load the film, take up the slack with the rewind knob/crank. The rewind will then turn as you advance the film, giving a positive indication that a) there is film in the thing, and b) it is advancing properly. Before opening the back, try the rewind to be certain that the film has either been rewound into the cassete, or is not present. Simple. Too difficult to do this? Get a digicam.