Note to self: when obtaining a free game from Humble, make sure to go all the way through to getting the key into steam promptly. Free keys expire (at least some of them do) if you don't do that.
I did the free 'purchase' of XCOM on Humble, but never collected the Steam key. When I went to do that over the weekend (a sale on XCOM at GOG reminded me), Humble had expired the key. Oh well - can't complain about losing something that I got for free due to neglect on my part. I just need to remember for next time.