the cities of santana do livramento in brazil and rivera in uruguay (heh heh heh, you are gay!) form one continious city - despite being in two seperate countries! customs checkpoints for both states lie outside the boundaries of the city/cities, and once inside you are free to move across the border and around the city as much as you'd like.
apparently it's possible to get lost in the suburbs and be unsure as to which country you're in.
this is what the 'border' looks like within the city zone: http://upload.wikimedia.org/wikipedia/commons/6/6e/Obelisco_-_Plaza_Internacional_-_Frontera_de_la_Paz_-_Livramento_-_Rivera.jpeg