Sungai Kolok, Thailand
We found out from someone on the bus that this was the "dodgiest border crossing in Thailand". We didn't really understand why until we got stopped leading up to it about 5 times by armed policemen that wanted to see our passports. I assume it was because they would have fined us on the spot it we had gone over our visa. Good job we hadn't, they had loads of guns, it was pretty scary!