I don’t know exactly when this weird phenomenon started. Turns out that whenever I try to copy a new version of an existing app into the Applications folder the transfer won’t succeed, leaving the app in a seemingly inconsistent state, indicated by a weird change of the app’s icon:
This may or may not have started happening with the advent of the App Store or, perhaps, when I installed Lion on my machine. Or else, this just as well may be totally unrelated to any of these “evolutionary steps”.
To be clear, downloads and updates from the App Store install flawlessly; I did not have any issues with App Store downloads, ever. The problem only occurs when I manually install apps.
The only remedy in this case is to delete the existing version of the app manually from the Applications folder and then start to copy the new version again. This time it works, of course.