This enemy ran up to my sniper on the final mission. I shot it, it died and trapped my soldier in place. Serious collision issue here that was never fixed. I Had the EXACT same problem on the final mission on my first play-through soon after the game came out,

Standing next to a Scylla when it does might result in parts of it overlapping the units next to it, resulting in them not being able to move at all. In all three scenarios this happened, the characters remained stuck even if the corpse of the Scylla was destroyed.