To be fair I never would have considered such a bug when using dropbox - I would probably have considered it safe considering you have a local copy of your data, especially since as other pointed out they present themselves as a backup service.
I almost got hit by this locally actually, I synchronize folders against multiple PCs, and the exact same thing happened, and a number of files had their bytes zeroed out and then this was propagated through the network. Thankfully I spotted it before all copies were overwritten and fixed it, but that's where you also want something like Time Machine.
Ugh, so many ways to lose data, even when you're doing the 'right' thing!