What is this bug

I had 257 more attack to recap me but his genral appeaar in 2 battle http://prntscr.com/5vgxog

At-war must hate me

Good question, has happened to me before. i remember trying a recap against an opponent with 35 imp inf and gen in my cap using 40 ds helis. I got something like that on the battlescreen and when i looked after i had lost and he had 20 inf remaining in my cap. We were both wondering wtf had happened.

This is a known bug that has cropped up a load of times.
From tests from back when I was r6, I think what happens is that one of the players is gathering units to send into the city right next to the city. However as stacks are being combined one of them gets turnblocked. However because of the auto-move into city thing the stack then tries to go to the city after it has already fought in a battle. A stack cannot fight a battle in two places at once, and so the game goes into meltdown and just kills the stack because it shouldn't exist.