[Mobhack]

I have had plenty of enemy tanks end up in the same hex as friendlies, as well as other enemy types than armour. If healthy they happily fire at range zero. So being in the same hex as the enemy isn't all that safe!.

As the original game didn't allow you to share the same hex as an enemy unit, firing at range 0 does sometimes bring about strange things, like the firing unit turning to face a strange direction, or the attack going in on an unexpected armour face. But those quirks have been there since SSI allowed hex sharing in a later build of SP1. Or if the track is damaged, then it can decide that it cant bear on a target inside its own hex.

My guess is that the tank in question was damaged (no weapon or out of ammo), or it was in a bad way due to suppression or morale, or the track were blown off and so it was not allowed to bring its weapons to bear.