There are better ways to spend the 4th of July holiday than answering scores of angry support emails, but that's apparently what several iOS and Mac App Store developers wound up doing thanks to a glitch in Apple's software distribution systems.
First reported by Instapaper sole proprietor Marco Arment and subsequently affirmed by other devs including Good.iWare (Goodreader) and Readdle (Scanner Pro), the problem seems to have affected apps updated on July 3rd forward. User symptoms are pretty simple: the app crashes, rather abruptly.
The good news is apparently only users who updated within the first few hours of a new version's availability are affected, and deleting and reinstalling the app generally clears up the issue; that removes any on-device settings and stored content, however. Goodreader's developers did a nice job of documenting the backup/app removal/reinstall/restore process, if you've got a corrupt app with onboard data.
As to the "why" of this problem, it's not clear yet, but the affected developers (more than 70 so far by Arment's count) are zeroing in on issues with Apple's encryption pass on the submitted executables. When an app is handed off to Apple for distribution in the App Store and approved by the review team, it's signed with Apple's digital keys (to allow iOS devices and Macs to run it) and encrypted to discourage piracy. The Goodreader devs suggest in their post that this encryption process isn't fully complete at the point that customers are getting a "new version available, download now!" notification -- so when they click that Update button, what they get is a half-baked binary rather than the correctly formatted app.
In the short term, there's not much for users to do except wait an hour or two before downloading new app updates that show up (kind of like waiting 30 minutes after eating before heading into the pool). We'll reach out to Apple for comment and see what, if anything, we hear back.