I've noticed that some types of cover in the game are completely indestructible. One example are cars. While you can blow a car up, it won't actually destroy it as a piece of cover that you or enemies can hide behind.
I've also noticed that some types of trees actually require more than one action to completely destroy as cover. An explosive will reduce the tree down to a stump (which still counts as high cover), while a second explosive will finally destroy it completely.
Are there any other types of unusual cover in the game that are effectively indestructible, or that require special actions to destroy (like attacking it more than once)?