Proving that the dev is usually better at their games than most players, I managed to softlock as early as round 16.
Same deal, also on Wave 16: one of the goblins kinda spazzed out as they were spawning and flew off the top of the map.
Also, if one of the grey(?) goblins steals your fighters, you can place another in their spot and effectively double- or even triple-stack them.
HA! Caught one of the little bastards in the act on Wave 2...6, I wanna say?
