This week, numerous users have reported problems with their Mac apps after Apple accidentally allowed a security certificate for the Mac App Store to expire. App users received error messages indicating that their apps had been "damaged." Apple quickly corrected the problem, but many users still had to delete and reinstall apps.
For developers, the problem was particularly frustrating because they were receiving complaints from users, but there wasn't anything developers could do to address the problems.
Apple has not yet issued a statement about the incident.