I agree with you about the game only letting a certain amount of zombies in at one time, but me and my mate did a glitch at level 10 (on verruckt) and by the time we had reached level 42 no zombies were coming into the building...my mate decided to come out of the glitch at 45 and still no zombies came in the building.
I then got the teddy bear in the box and it went down to the German side start room, we went down to get weapons and all the zombies stuck outside on the german side came in...but the weird thing was that as soon as we ran back into the American side they disappeared...
I have no clue why the zombies would not come inside the building, but I can tell you one thing...its good for getting points

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