Unless, as happened here, Dropbox erased the file and synced a blank version across all your computers. They they're safe and sound on any computer linked to your dropbox connection that hasn't been connected to the internet since the file got corrupted.
If you catch it in the 30 day window when they keep old versions you're find, but there are files in my Dropbox that I don't use every 30 days.
On the other hand, I don't want to immediately blame Dropbox just yet either. If you backup garbage (say, because you have disk corruption), then you can't blame Dropbox for backing up exactly what you told it to.
And Dropbox does offer a premium Packrat service if you want file history indefinitely. Perhaps the user can be blamed for assuming that he/she would only need 30 days of history, but this is really contingent on who caused the corruption to happen in the first place -- and that's unknown at the moment. [1]
A person running Time Machine (or similar incremental backup system) is a lot safer from this sort of problem than a free tier Dropbox user. Free Dropbox is better than nothing, but people can't keep assuming their files are safe because "they're in the cloud" and get synced to a few places.
Please stop. Right there. Stop believing some vendor marketing blindly (even if you do come to another conclusion later), stop reassuring other people who do so and stop calling a sync a backup. There is a very important distinction: Sync has mechanisms in place that are capable to touch the files on your backup. At least in dropbox' case these mechanisms are not completely separated from the initial backup mechanism of each version. I've had two almost catastrophical data losses with Dropbox until I was able to make that distinction. I've come to the conclusion that IT professionals should never ever treat a sync system as a backup system, and if you still think so please don't spread that advice to others.
Most of us here are developers. How many of our products are perfect? Always assume something will fail in a new and interesting way in the future. I'm not letting Dropbox off the hook - their product shouldn't do this - but you'll be happier if you treat backups with the same level of redundancy and planning as the rest of your infrastructure.
"Even if your computer has a meltdown, your stuff is always safe in Dropbox and can be restored in a snap.
In fact, if you're using the Dropbox desktop application, your files are backed up several times. The primary copy on your computer's hard drive is synced online and that copy is then backed up again for safety (emphasis mine). If you are using Dropbox to sync files between multiple computers, your files are backed up on those computers as well. If that isn't enough, Dropbox also keeps backups of all of your deleted and changed files too.
...
It's hard to imagine a scenario where Dropbox could lose your files. Hypothetically, let's say a nuclear bomb blows up the data centers where your files are saved. Even then, your files are still safe and sound on your computer and any other computers linked to your Dropbox account."
Clearly, though, Dropbox appears to have lost files (at least if we take it on faith that "these are bugs in Dropbox's syncing logic"), despite the fact that I see no mushroom cloud nearby.
[1] https://www.dropbox.com/help/122/en