ZDoom

Careful: PostBeginPlay is "called before the thinker ticks for the first time". Read this sentence very carefully to understand your mistake. This does not say "is called before the actor does anything else". In your case the missile is exploded right in the spawning function, and that happens BEFORE it can tick for the first time.

class BuggedPlasmaBall : PlasmaBall replaces PlasmaBall
{
	States
	{
	Death:
		TNT1 A 0 BugDeath;
		goto Super::Death;
	}
	
	override void PostBeginPlay()
	{
		A_Log("PostBeginPlay");
	}
	
	void BugDeath()
	{
		A_Log("Death");
	}
}

PostBeginPlay is expected to be executed before any of actor frame actions, right? But if you stand in front of a wall and fire a projectile, then the code in it's first death frame action is executed before PostBeginPlay.
Use the code below. Start the game, change you weapon to plasmagun, run to nearby wall as close as you can, fire the projectile. You will see "Death" message appear before "PostBeginPlay".
[code]
class BuggedPlasmaBall : PlasmaBall replaces PlasmaBall
{
States
{
Death:
TNT1 A 0 BugDeath;
goto Super::Death;
}

override void PostBeginPlay()
{
A_Log("PostBeginPlay");
}

void BugDeath()
{
A_Log("Death");
}
}
[/code]