I had a similar mishap last week. I'm developing film all the time and don't tend to have any reel loading problems, but there was this one roll of 120 where I hadn't noticed that the film wasn't securely in the clip, so it got pushed in too far, causing the two frames in the center of the reel to overlap--of course these were the two frames in the whole batch that I really wanted. I probably would have noticed at the end of the reel that it was in the wrong place if I had used a 120 reel, but loading 120 on a 220 reel, it's harder to predict where the end of the film should be on the outside.