At the beginning of the alien turns, just after I threw a stun gas grenade at them (and the gas actually covered them). Once I had a grenade land in front of them, and then it CTD when they ran forward into the stun gas. Honestly, it was so frustrating that I only played 4 games until I got to the T2 aliens, twice I got Sebillians, and twice I got another alien I didn't recognize (tall and lanky, one was holding a shield).