I just ran into another very fatal bug. I encountered it twice. I think both are related to this one. However, there are some differences.
First time:
I'm not sure what I was doing at the time. However, I may have accidentally tried to open a QST file in the tile grabber. I didn't know that beta 13 is supposed to be able to do that. Whether that was the cause or not, the quest started displaying gray screens for all of the squares in the screen selector. I couldn't change maps or screens. At the time, I thought that it was 100% frozen, so I saved it as a new file, closed it, and reopened it. The file from after the bug turned out to be unusable; it was in the same state that I had last seen it. I abandoned that file and went back to a previous save.
Second time:
This may have also been caused by the same thing, because I tried to rip some graphics from a QST file, this time intentionally. The problem did not occur immediately, but when it did, the map display got stuck on map 1 and practically everything froze.
Unfortunately, as the only way to tell something was wrong was to try to click on things, I did not realize the problem before saving the file. The resulting file is unopenable; it crashes instantly, whether by way of layering or otherwise. However, the auto-backup is readable.
When I opened the auto-backup as an imported QSU file, I found a mangled quest file. The subscreen data was warped (thought that may just be because it isn't yet support for imported QSU files). Perhaps more significant is that the map count had been reduced to 4, rather than 1 (or zero, even). Isn't that the default number of maps in blank quest template?...
Aside from lost data, the backup QSU seems to be a functional QST file. I am obviously hesitant to use it, though. The only changes it contained were combos and tiles, thankfully.
I will hold on to the two mangled quest files. If one of the developers would like to see them, I can send them via email or something.
P.S. -
I use Windows XP Home Edition, if that matters.