I just played a game as Graves, and at one point the enemy team broke two of our inhibitors. I got one of theirs as well shortly after.
Right after this, two super minions started coming down the two lanes without inhibitors (mid and bot), every wave came with 3 melee, 3 caster, and 2 super minions. This pretty much lost us the game, and was very frustrating. We still had our last top tower and the inhibitor until the very end, yet the two supers per wave continued.

After a certain amount of time (30 minutes? 40?) a downed inhibitor will result in two super minions each wave, or six times the normal amount. This is to break stalemates.