Lost Souls can lose their properties
Under certain specific conditions, Lost Souls can lose their properties entirely. Some of the side effects include: A Lost Soul becoming a ghost, requiring splash damage to harm them, or becoming completely invulnerable, losing their flying properties, but having "semi-clipping" properties.
The only way to trigger this bug is through the use of a DeHackEd file.
To cause the bug, modify the bullet puff in DeHacked to use rocket explosion frames, with one hit point giving it a splash damage value. Next, modify the blood splat thing to have one hit point, with the death being the explosion frame, with a splash damage value. This will create a crude "explosive bullets" patch. While this works in most conditions, Lost Souls may randomly exhibit this bug's behavior if shot.
The supposed cause of this bug is due to the fixed amount of states that can be changed. If there are too many state changes at once, so the game will either apply the properties of the actor that harms the monster to the monster itself, or the monster will be stuck in a perpetual state. The Lost Soul in this case is only killable by being crushed from a door or crushing ceiling.
There were a lot of random DeHackEd patches released, some being very simple, like changing ammo values, to being ambitious, such as changing properties and pointers of things, including bullet puffs and explosions. A lot of these were done without thought to how Doom would actually perform or behave in certain situations.
The reason for this being significant is that the lost soul is the only Doom monster that can be affected by this bug in normal play. Other monsters will simply enter a pain state or die.
In Source Ports
All source ports that support DeHackEd files may exhibit this bug. In The Eternity Engine, you can create a scenario where an "invulnerable" Lost Soul can attack a friendly monster, causing the friendly monster to retaliate by attacking the player.