Admit it. It happened to you before and you lost time and effort redoing work you didn’t save.
In my case, it happens because I get caught up in the intellectual process and forget that every time I delete a reference entered through a reference manager, the word editor might freeze and crash (I am obligated to use windows at work).
So a tip for myself before everyone else, make saving a habit. Train it in the beginning by setting a repeating 5 minuet interval reminder to save. Routinely backing up is also a must because servers, user accounts, computer hard drives are damaged constantly.
The ultimate sin, however, is to have the only copy of your thesis on a USB drive. Even more, one that has other files and you plug in several computers for different purposes. Avoid that at any cost!