Funny that you mention that, because ever since I've had this problem hl2.exe has been crashing upon exit. However, I have the same bug with TF2, yet I can start it up at anytime. I only get this particular steam error on INS. Furthermore, I've noticed another strange thing which is probably more related than the hl2.exe bug. When I use steam chat it says I'm in-game even when I'm not. So perhaps steam is not registering that I have exited INS; therefore, it won't let me open up a "second" instance.