A couple of weeks ago, Play Store users started receiving frustrating errors when downloading or updating apps. The bug flashed a dialog reading "Package file is invalid" after refusing to download an app. It seemed to affect people and apps randomly. Google marked the issue as "resolved" on the Play support page on August 13th, but it has been moved back into the Known Issues list.
A casual search on the social networks reveals that more than a few users are still experiencing the issue, though it doesn't seem to be as widespread as before.