This should technically belong in the bug forum but me thinking that this hasn't been fixed since I started playing this game a few years ago, suggest maybe OOO have decided it's ok for the odd zombie to get pushed through a solid gate and we lose those valuable coins and materials.
If you don't known what I mean here is a brief description.
Start The King of Ashes mission (or FSC as we 1st generation players like to call it). Arrive at D24 and stand on pad. Now this doesn't happen all the time but it can happen, a zombie getting pushed by a wheel will get pushed through the gate behind us.
Please fix, if this is a bug.
I've seen zombies get pushed off the map by the wheels, so long at this bug doesn't prevent you from continuing then it is a non-fatal bug and the loss of a few CR can be overlooked.