Slitherine

I was wiping out a faction, when I got snookered by someone else (they conquered the last city of the dying faction). My troops ended up behind their lines (even though I had a non-aggresion pact with them), and even after I declared war on them, my units couldn't attack them for effect.

The units affected now have "nan" over them, and are effectively invulnerable to each other...and every nan unit that attacks another nan unit makes it a nan unit...so it's contagious.

Here's the save from a few turns before.