The primary way we’ve had trouble crossing the borders is due to incorrect paperwork. What happens is at one border someone fills out something incorrectly or neglects to fill out something.
Sometimes this is done on purpose. The problem arrises when you reach the next border crossing and they discover the ‘problem’.
I had an interesting insight into one reason this happens. At one of the many windows where you get some of the many papers stamped I came across a pretty nice guy that hadn’t a clue what he was doing. He was an ex-banker and was plenty smart enough and willing enough and honest enough to do the job.
His problem was that nobody had actually sat him down and explained the process. So his solution was that leverage the more experienced guard. Imagine a fellow with a sawed off shotgun whispering out of the corner of his mouth ‘no, no he gets the pink copy and you file the green’.