Death Hill Cobra ability
My opponent was attacking and it was the declare blockers step. I had a Death-Hood Cobra with Reach. Each time I tried to select the Death-Hood Cobra to block the game brought up the option to select one of the two abilities and I couldn't assign it as a blocker.
I know I had a small chance of victory prior to this point, but after not being able to select the cobra as a blocker I feel that my chances of winning went to nil. I conceded the game after this step and this was the second loss in the match so I effectively conceded the match as well.