ZDoom

Fixed in [url=https://github.com/coelckers/gzdoom/commit/12eb760ff4905196a54d8b1196760f4e176be797]12eb760[/url]. By the way pre-ZScript versions just crashed in this case.

[s]Load the file with Hexen and go to MAP40. Enter the first chamber where Korax* is. Summon TestDemon1. It's a friendly chaos serpent which disappears at the end of its death sequence. Let it and Korax fight each other, and if it dies and disappears while Korax is in its attack sequence, you'll get the abort.

[attachment=0]korax_bug.pk3[/attachment]
* This is a modified Korax, made to make it easier to produce the problem.[/s]

Edit: For convenience, I added this to the first post.

Is there ready to use sample to reproduce this issue quickly?

[b]Tested with: 3.2.5 (64-bit)[/b]

Check [url=https://forum.zdoom.org/viewtopic.php?p=1041050#p1041050]this post[/url]. It might be very difficult to make it abort with the original Korax class, but if you had a modified one with a code like this for instance:

[spoiler][code=php]class TestKorax : Korax replaces Korax
{
    States
    {
    Attack:
        KORX E 4 Bright A_FaceTarget;
        KORX F 8 Bright A_KoraxMissile;
        KORX E 4 Bright A_FaceTarget;
        KORX F 8 Bright A_KoraxMissile;
        KORX E 4 Bright A_FaceTarget;
        KORX F 8 Bright A_KoraxMissile;
        KORX E 4 Bright A_FaceTarget;
        KORX F 8 Bright A_KoraxMissile;
        KORX E 4 Bright A_FaceTarget;
        KORX F 8 Bright A_KoraxMissile;
        KORX E 4 Bright A_FaceTarget;
        KORX F 8 Bright A_KoraxMissile;
        KORX E 8 Bright;
        Goto See;
    }
}[/code][/spoiler]
It becomes easier, provided the target somehow ceases to exist during the attack sequence (think of a monster that disappears completely after death, like the Lost Soul).

Edit: Here's something to test: https://www.dropbox.com/s/x28gf610dm1uioa/korax_bug.pk3?dl=1

Load the file with Hexen and go to MAP40. Enter the first chamber where Korax* is. Summon TestDemon1. It's a friendly chaos serpent which disappears at the end of its death sequence. Let it and Korax fight each other, and if it dies and disappears while Korax is in its attack sequence, you'll get the abort.

* This is a modified Korax, made to make it easier to produce the problem.