Taking a screenshot of a notification breaks the PrintScreen button
Solution 1:
I couldn't find a bug report about this issue in Launchpad, so you can file a new bug report against the gnome-screenshot package to let developers know about it. Read this question to know how to file a bug report: How do I report a bug?
When PrintScreen becomes unresponsive, restarting GNOME Shell will probably restore its functionality. To restart GNOME Shell press Alt+F2, type r, and hit Enter.