I keep loosing the ability to melee intermittently. I think that it's resolvable if you restart the game but I never really needed melee so I didn't bother(finished it now), I just keep finding when I come back to the game I can always melee to start with. I suppose it could be my keyboard or something if nobody else noticed it.
I noticed this last night when I was close to the end of the game in a crucial battle with the Ceph. It happened earlier in the game, but I thought it was just a fluke. Another thing that needs to be patched. Still an awesome game though.