A Microsoft file hosting and synchronization service.
I have this problem even with files that I open on only one machine, which is kind of crazy.
Today I noticed that this happened with a Word document and an Excel spreadsheet. Both are usually open in the background, with auto-save enabled, so they had more than enough time (hours, if not days) to sync.
Just because I was lucky enough to notice a missing paragraph that I knew I had written in the Word document, I decided to investigate. In version history, I could see that on September 30, the two files were overwritten with versions from September 27.
What's even more disconcerting is that I hadn't opened or worked on the documents at all between September 27 and today, October 23. So this wasn't some kind of connectivity issue or sync problem – OneDrive decided to overwrite my files with older versions all on its own, without any indication to me. I'm now pretty worried about how many other changes, and which ones, I might have inadvertently lost in my thousands of other files. Even if I could check them all, I don't know their latest contents by heart (which is kind of why I use a computer to store the stuff, and not my brain).
I guess you could find these cases by checking the version histories for changes that happened without the files having been opened, but without some automated way to do this, that's not feasible.