A group of us were taking a car to the airport to head home at about 4 am. We were driving down the road and a policeman stepped into the road with a rifle and pointed it at our car and waved us to the side of the road.
When he came and looked in the car, he told us (in Spanish) that if we didn't each give him $400 USD he would confiscate our passports and throw us in jail. We were eventually able to talk our way out of it by telling him we were missionaries and didn't have that type of money (which was kind of true, 2 of the people in the car were medical volunteers in a small village a few hours outside SPS).
After about 5 minutes of arguing, he waved us on. It was a pretty tense few minutes and our driver told us it was very common. Again, this was a policeman doing this, in the middle of a fairly major road.