I noticed that in R24765 (I love Ctrl+V), when I kill an alien, they stay in the animation they were performing at the time, IE, if it was walking and killed by reaction fire, and you kill it, it stays on that square, with the walk animation. They are still holding their weapons, though they also appear on the ground. Would be a very minor bug, except the game won't let you move through that square. Once you start your next turn, they return to normal, dead aliens.
I also noticed that the little alien counter in the top right will go into the negatives when I kill one. It was... odd...
Finally, several times, I've wound up ending my turn looking at an alien several squares away. When they start, they perform their melee attack animation, but start moving (looks like their floating) while they do it. My man, of course, dies. I can't tell if it's just doing it out of sequence, but it seems like they kill my guy, then move away, as if they didn't spend the TUs getting next to him.