Need more information, like how to reproduce it, what game you were running, and what distro / system you have.
That's the funny part, for me, when it happens, there is nothing special to be done before. It just doesn't work e.g. immediately after opening web browser or chat in overlay. Additionally, in web browser, even Enter is not working in URL bar. But then, at some point, something changes and everything works like it should.
I'm not really able to give more details or steps to reproduce at this point. :/
The overlay might think the control key is down (or shift or alt). When it happens, can you try pressing and releasing those keys, then try your keystrokes again and see if they work.
We also need to know what game you were running when this happened, what distro, and what window manager you are using.
Thanks.
@slouken: You we're right. This odd behavior is connected with alt+tab, occurs when I alt+tab outside from the game and then back. Pressing alt seems to restore the correct behavior.
System info + running CS:S on Gnome Shell 3.6 & Debian x86_64.
Nothing extracted yet.
I'm sometimes experiencing some strange problems with overlay. Last time and now, CTRL+C, CTRL+V, CTRL+A and backspace were not working at all, neither inside overlay, nor outside (in other apps).
System info