One possibility is part of the film skipped the track. If you do this in daylight (with junk film of course) and verify every part of the film is in track, does it still happen?
If it does, with the camera you normally use, how close is the last shot to the end of the film? Mine ends up pretty close to the taped area. If this is the case for you also, then you could potentially wind from the end. Typically, there are quite a long leader area at the beginning. If these gets unevenly processed, it wouldn't be a big deal. The other way around may be a problem.
I've never experienced this personally; however....