I can't quite put my foot down on what the scenario is exactly. One time might have been a case where a folder with lots of small files got moved away and Dropb

I can't quite put my foot down on what the scenario is exactly. One time might have been a case where a folder with lots of small files got moved away and Dropbox only allows single file restore, which would have taken hours, possibly longer than the lost work. Once I've done something stupid where I wanted to exchange a whole Dropbox folder with a different version. I switched off the client on one PC (by mistake, obviously) and exchanged the folder on the other, then switched it on again. Then I became aware the the old folder had important stuff in it. The versioning was corrupted at that point, many old files would either not show up online or would produce an error message. So usually there is some user behaviour involved, yes. But the whole point of a backup system is that you can rely on it, even when the user behaves stupidly up to a certain degree. Sync is for day to day collaboration and data management, it's not for backup.