Okay, so I have this pet... Little Low-Tier Timmy
I leeched him down to 0, so his health was temporarily 10/0.
I then attempted to take him into a battle, because why not. It let me choose my two T1 weapons and start the battle. At which point, I hit this:

You see, undead Little Timmy appears to be so powerful that he resurrected the old coliseum. I didn't get a chance to battle once before the old buttons appeared. Neither button did anything upon click. When I manually went back to the BC, it didn't register that I had an ongoing battle. But Timmy is now at 0/0 HP, and has a loss on his spotless record.
(Also I've altered my BC view with CSS. So it might look slightly different if one didn't have it. But still the buttons are wrong (should be "See Results"), there shouldn't be 2, they should go somewhere, and it should've have let me battle.)
During the gelly war i ran into this issue probably around 3-5 times, where it would say game over instead of see results, then would sort of lock me from doing anything. no idea if the damage i had during those turns/the battle itself counted, but my pet is completely normal so no reason it would just be the 10/0 leached pet thing.
Have either of you encountered this error recently? :O
The only time I ever saw this happen is when I leeched my pet to 0hp. I haven't leeched a pet since this post, and don't plan to again. So I really cannot confirm the current state of things.
Perhaps a dev could set a pet up to have 10/0 health for easier replication?