1. Create an huge file (e.g. 1024x1024 with 64 frames)
2. Save the file into a .ase
3. While it's being saved, close the program (Ctrl+Q)

There are two critical issues here:

1. If the document is modified, Aseprite will ask us to save the changes (yeah, while it's saving changes and the progress bar is working)
2. If the document isn't modified, Aseprite will not ask us anything, it'll close itself and the saving process canceled in the middle, losing all other (non-saved) frames.