Honestly, in the Amazon warfront zone in Firefall there was a corner of the map where enemies would just keep spawning indefinitely. I think it had like a counter that if an enemy got destroyed, shortly after a new one spawned, creating the illusion of an endless wave.
Didn't serve much purpose but it was fun. XD
(Edit; They kept coming through the melding, making it visually pretty consistent with a never-ending invasion)