Some users claimed to have seen files that were deleted six years ago back in their folders, which shouldn't be possible according to Dropbox's own explanation of its deletion policy.
Users can recover files themselves for 30 days, but Dropbox may keep files marked for permanent deletion on its server for a further 30 days. After 60 days, users on its basic plan aren't supposed to be able to recover files. Only business users can recover files after this period.
However, due to a series of missteps, some basics users saw gigabytes of old files returned from several years ago.
"It just made my server upload 4GB of unnecessary/obsolete files for no reason," wrote one user who saw several folders restored from 2011 and 2012.
Dropbox employee Ross S explained on Thursday that a bug stopped some files from being deleted from its servers and that while fixing the issue the firm had accidentally restored those files.
"A bug was preventing some files and folders from being fully deleted off our servers, even after users had deleted them from their Dropbox accounts. While fixing the bug, we inadvertently restored the impacted files and folders to those users' accounts. This was our mistake; it wasn't due to a third party and you weren't hacked," he wrote.
Deleted user files were not actually being removed from Dropbox's servers due to "metadata inconsistencies" in affected files, according to the firm.
"Typically, we permanently remove files and folders from our servers within 60 days of a user deleting them. However, the deleted files and folders impacted by this bug had metadata inconsistencies. So we quarantined and excluded them from the permanent deletion process until the metadata could be fixed," said Ross S.
Some users continued to see restored files in their accounts after Thursday. Dropbox on Sunday said it was rolling out a fix that would automatically delete the affected files, unless users had changed the files since then. Users had 30 days from Sunday to request the files be restored if they decide they actually want to keep them.