For me it would have been my worldspace that I have, in fact all of the screwed up versions that are broken are still there in the mod as a reminder.
The first iteration was cool and all, almost finished it and then it corrupted, made sense though since it was my first attempt so I wasn't too mad.
The second iteration I am still a bit upset about though.. that one took longer than the first, looked a ton better than the third (a TON of more time too) but it ended up having a corruption when trying to generate textures, it was my favorite.. Wasn't until the third that I ended up learning how to backup worldspaces after a "Alpha" upload on the Nexus.
Third iteration managed to get finished with some minor setbacks time to time. I always backed this one up to prevent it, but every time I would generate textures on this one the same thing that happened to the second happened causing complete file corruption having to use the backup. The only other time it screwed up was when I was making a hub to transport between cells THAT decided to corrupt on its own also making me have to revert a character save back about 5 levels or so (remember, HARD SAVE prior to entering a test cell and don't use quicksave) and putting me back about a week.