Every once in a blue moon, one of my peons (sometimes, along with an enemy peasant) would get stuck near the lower-right corner of the screen.
Example Screenshot:
Example Match URL:
Ah, I think the ruined tower in the lower right was covering a gem or something, and it was just slightly too big, so you couldn’t grab the gem from behind it. I’ve shrunk it a bit, so you should be able to get it now.