When trying to start Portal 2 Steam starts completing the installation, aborts after a few percents and then gives the error message
Incomplete installation of Portal 2 (10)
I've verified the game cache, but that did not help. Any ideas?
Apparently that message only means the Steam servers are busy, it worked now.
Incomplete Installation (10)
Steam servers are busy. Please try again later.
When I got this, it was because my computer had gone into hibernation during the download. A Portal 2 update came a few seconds later, fixing the problem. Verifying the game cache was unnecessary.
Switch to another download region in your Steam settings under Downloads and Cloud tab. I switched to "South America" and got a server. Had to try a few different non-US countries to get it to work.
Wait a bit and run Portal 2 again, for me it started the installation again and completed the second time.
Your mileage may vary, let me know if restarting Steam and rebooting your PC have no effect either...
According to fail badp ♦, Steam servers are currently too busy to handle all those installation requests.