Many Mac owners have found themselves forced to delete and re-download Mac App Store apps after what appears to be a certificate expiration error caused the operating system to mistakenly identify the apps as damaged.
The errors variously range from "this app is damaged and can't be opened" to "this app was purchased on another computer." In some cases, users were able to rectify the problem by logging out of and back into the Mac App Store or simply restarting their computer.
In many instances, though, the apps had to be deleted and then re-downloaded. Tweetbot developer Paul Haddad was the first to diagnose the problem as an certificate issue.
Each of Apple's App Stores require apps to be signed with a valid certificate. This practice allows for increased security — malicious apps can have their certificates revoked, for instance — and helps to guard against piracy.
Apple has yet to comment on the problem, though it does not appear to have recurred for users who resolved it once.