I have two creatures with Fight or Flight (Valkyrie Scout plus Ugat with it on an artifact). This ability makes an attack against something that attacks a team member.
I was fighting an enemy that cast a spell on my valkyrie that I believe made it attack another of my own creatures - I think the other one with Fight or Flight.
I couldn’t check the combat log for details on what happened next as the two seemed to be caught in an infinite loop attacking each other (and you can’t pause to check the combat log). The fight ended as the valkyrie had some spells that cast on being hit and those targeted the enemy side, eventually wiping them out after many rounds of the two team members attacking each other.
If it at all helps/is needed for reproduction I also have a creature with fan of knives and that damage would proc as the two attacked each other as well.