Spoiler:The spheres' behavior consists of chasing after the targets and exploding in their faces through their (the spheres) melee state. They can also be shot to explode, as well. In addition, they sometimes, while chasing, "teleport out" and then "teleport in".
The problem: If you take a look at the code above, the spheres become unshootable and unsolid when they're at the beginning of the teleportation sequence, when they teleport in again, they become shootable and solid again. In normal circumstances that sequence cannot be interrupted, so there is no way for the spheres to roam around in a visible, unshootable and unsolid state while chasing the target. It's the same thing with their melee state; once they enter the state, they'll become invulnerable, play the sequence, become vulnerable and die (detonate). And just like with the teleportation sequence, there is no chance for the melee state sequence to be interrupted under normal circumstances, and thus you won't have roaming invulnerable spheres, naturally. However, despite that, they sometimes can "break" this and become invulnerable and/or unshootable in other situations.
Through testing and observation, I found out that they only exhibit this strange behavior while flying after being spawned by the Overlord but before exiting the "state" A_SkullAttack puts them in (note that this doesn't happen 100% of the times, but will eventually). My "theory" is that they try to execute these sequences (melee and teleportation) while in that state, but don't succeed entirely (they fail to complete the sequences fully), due to them being in that A_SkullAttack-induced state.
According to the wiki on the [wiki=Actor_flags#SKULLFLY]SKULLFLY[/wiki] flag (which is set by A_SkullAttack), a monster with this flag will not enter its pain state. Now, could the flag has something to do with that peculiar behavior seen on the hade spheres here, in the sense that it's, somehow, affecting the sphere's ability when it comes to changing their states?
