All you need to do is pull the back away from the camera body slightly, even with the darkslide moving while you're inserting it, to get this. If the change in density does not show up in the clear film rebate, that's likely what happened.
If the rebate is also fogged then look for an out-of-camera cause, e.g., light leaks in the darkroom. The film would then have been partially covered by another sheet when the exposure happened, hence the line. This can even happen in the fix if you're using an alkaline fixer and haven't given an adequate stop and then turn on the lights too soon (don't ask me how I know
Best,
Doremus