Joe Rossignol, writing for MacRumors:

A growing number of MacRumors readers and Twitter users have been experiencing an issue with some Mac App Store apps displaying a “damaged” error when opened since late Wednesday. The issue appears to be rather widespread, affecting popular apps such as 1Password, Acorn, Byword, Call of Duty 2, DaisyDisk and Tweetbot.

I’ve experienced this first-hand, albeit without the “damaged” dialog. Tweetbot was running fine on my El Capitan Mac. But when I quit it and relaunched, I was prompted to re-enter my AppleID password. Hopefully, this gets resolved quickly.