Originally Posted by Pioneer
You can rewind the film in a darkroom and start over if you are patient.
Well, sort of. 120 is fixed on the wrong end, so naive respooling will end up with film with a considerable bulge and corresponding transport and light leak problems. The one case where you can avoid bulging is when the film end is still firmly wrapped on the original roll - so you can carefully wind back if there are still several (four 6x7 or equivalent) frames left to expose and you do not unwind any more of the film. Do not try to do it in-camera - medium format transport mechanisms are not designed to go backwards and are very likely to break.
Once the film has been transported past the last frame, you have created a offset between film and backing paper. You can only undo that with a very accurate template and lots of rehearsal (or IR goggles). Or you can carefully detach the adhesive tape from the backing once you have wound it up to the bulge, and reattach with everything lined up - but you may lose (or overlap) a frame over that as that displaces the film.
In any case, you may end up with film bulging past the spool, and some backing papers will tear rather than detach. You should be prepared to switch to immediate processing - you cannot hand on a damaged roll to the average lab and can't even store or transport it if it does not fit a light-tight 120 canister any more. And even if your respooling attempt seems to work out, it might come apart or crease in the camera, and cause expensive shutter damage.