A Dropbox bug has left a number of users with missing files that may never be recovered. The issue, which affected those who enabled the Selective Sync feature, is now fixed — but Dropbox has conceded that it may not be able to rescue everything that has been lost.

“Unfortunately, some of your files were deleted when the Dropbox desktop application was shut down or restarted while you were applying Selective Sync settings,” Dropbox told users in an email today. “Our team worked hard to restore files that were deleted from your account.”

The hiccup gives credence to the warnings that say a cloud-based backup should never be your only backup. Engadget reports that a number of users “ found that they’d lost years’ worth of content through no fault of their own” due to this issue, and without local copies of that content, it may have just disappeared for good.

There is some good news for those who were affected, however. Dropbox says that it’s putting additional tests in place to prevent a similar situation from reoccurring, and it’s offering everyone who had files deleted a year of Dropbox Pro for free starting today.

You’ll get an email if you’re one of the unlucky ones, and it will include a link to a “personalized web page” that will tell you exactly which of your files was deleted, and whether or not it could be recovered. If you haven’t had an email and nothing appears to be missing from your account, you should be okay.

• ViaEngadget