I just finished a game on Steel Peel and there was a Firebase placed by the opposing team that would not take damage. I was playing as a Tank and the enemy Firebase was place on a tiny ledge near the center of the arena behind glass. My Product Grenade was capable of stunning it, but nothing I did seemed to damage it. Jet Gun? No damage. Death Blossom? No damage. Product Grenade? Stunned, but still no damage. Rail Gun wasn't really an option since I couldn't get an angle on it. The Support would overheal it, but I never saw its health go below its normal maximum when I would attack it. This was definitely not normal. I don't know if it was immune to Airstrikes as our Supports were busy helping to defend our base. Neither Ejector seemed to effect it either.
I would blame it on lagg. I remember a strange spot on Ammo mule where you clip the firebase onto a wall which would make it invisible and very hard to hit but still destroyable.
