I am using the Dropbox desktop app on a Mac. Twice now, I have had files deleted out from under me, literally as I was still working on them in Word. In place of the filename, it shows up as a weird temp file. Once I closed it without realizing and thought I had lost the file completely, but was able to restore it by undeleting it — though I had never deleted it (or even closed it!). The second time, I resaved it under its original name — after seeing that it was gone from its folder.
This kind of glitch is completely unacceptable and makes me no longer trust Dropbox. A cloud backup system should not delete files at all, much less while I am actively working on them.