Enemies getting stuck or confused.
Physically touching an enemy will immobilize them. Other enemies get stuck behind the first. Shots (from you or other enemies) pass through the enemy touching you (but will not pass through the ones who are stuck behind the first). If you move away from the enemy, they will start moving again.
Sometimes two enemies will be on opposite ends of a box. They stop moving until one is killed.
Sometimes I see enemies on top of mountains who seemingly cannot get down. I also cannot shoot at them.
Also, enemies getting stuck behind trees and fences.
And sometimes enemies touch me and I instantly die. I don't know what happens.
On the first part, when I said your shots pass through the enemy touching you - this only applies to the machine gun. The enemy is still invincible. i.e. The rocket launcher will explode on the enemy next to you, but will not hurt you or the enemy.