As an example of a really porous border, the German/Swiss border. I used to live right at it (there are 3 bridges in my town leading into switzerland). There's

As an example of a really porous border, the German/Swiss border. I used to live right at it (there are 3 bridges in my town leading into switzerland). There's one which you can only walk/bike over which used to have a border control which is now deserted. The other 2 ones have border controls, but they seem to be solely tasked with checking whether cars are allowed to drive in switzerland (road fees), whether swiss people are buying more stuff in Germany than they're allowed to (the price differences are immense, so we've got people comming from as far as Zurich to shop here), and sales tax refunds (Swiss people buying in Germany don't have to pay the Mehrwertsteuer of 19%, but the significantly lower Swiss sales tax. As the MwS. is factored into the purchase price, it's centrally refunded at the border). It's more of a tollbooth/administrativa affair, really.