During the game what sometimes occurs is a monster turns invisible, while fighting a spider it kicked my sovereigns butt, 

so after I went back to the same territory the spider was no longer there but clicking on a tile my cursor turned into the sword icon and I fought it again only this time it was invisible ( I had no idea what I was fighting, but it turned out to be the spider).

It also seems that the champions often simply vanish after they go through fog of war (this one is pretty common, it happened the most frequent.).

What's strange is that after they go through the fog of war, you can send your sovereign there and there is nothing in site. 

I have also had buildings in a city disappear, so I couldn't tell what was where anymore.

hopefully that was descriptive enough.