In defense of the OP, when I made the border crossing at Foz do Iguacu last week, the border was completely unguarded. No gates, no officers, nothing. If you didn't know better (I didn't), it would be very easy to pass by it without anything to stop you.

Fortunately my taxi driver knew that he needed to park the taxi so I could get out and go into the office at the border crossing and complete the formalities. For locals (such as those most likely to be crossing using a public bus), I suppose no formalities are necessary, because my taxi driver didn't even have to show any ID to enter or exit Brazil.