The boundary between India and Bangladesh (formerly East Pakistan, as on this map,) is really, really complicated.
Notice the enclaves within enclaves.
At one point, it even does this:
Yes, that is a piece of India, that is inside a piece of Bangladesh, that is inside a piece of India, that is inside Bangladesh, known as Dahala Khagrabari #51.
Do they even bother with customs and border guards?