Cautious types will frequently tell you not to rely on cloud storage as your only backup, and a handful of internet denizens have just learned this the hard way. Dropbox has confirmed that a bug in some older versions of its desktop apps deleted the files of some people who turned on Selective Sync, which limits cloud syncing to certain folders. Typically, this would happen after a crash or forced reboot, making a bad problem worse -- at least a few users found that they'd lost years' worth of content through no fault of their own.
The good news? Most of this (though not all) is now past tense. Dropbox says that it's restoring files when it can; it's also releasing fixed versions of its apps, preventing older apps from working and putting extra safeguards in place to prevent this kind of error from happening again. Affected users are getting email (such as what you see below) that offers a year's worth of Dropbox Pro service for the trouble. While these efforts won't be much consolation if you lost access to files at a critical moment, it at least shows that Dropbox wants to learn from its mistakes.