The dangers of not having a visa

Sometimes people say to me I am only trying to sell them a visa, and not really giving them useful travel advice. At first glance this may seem fair criticism as I do make a living selling visa services. It is also possible in some places at some times to buy cheap (or at least cheaper) visas at the border. But is this really a good idea? No.
Be prepared. Are you really willing to risk travelling half way around the world without that visa just to have saved a few dollars, then find out you cannot buy a border visa upon arrival? The wise traveller has all their visas and documents in hand prior to getting on the airplane.
The following story happened to people with whom I was travelling on a Mekong River cruise from Cambodia to Vietnam in 2005. As the boat sailed from Cambodia into Vietnam, each passenger had to have a valid Vietnamese visa in their passport. These were all collected at the river border post for checking and stamping. One couple had bought a common single entry visa for Vietnam a few months before. This would have been fine had they not already used it! Yes, they landed in Vietnam, then flew on to Cambodia to catch the river boat. Now they were trying to re-enter Vietnam with a used visa. Entry was denied to the entire boat, and we floated on a watery border no-man's-land for 18 hours as one crewmember took a speed boat then a motorcycle back to Phnom Penh to get two new Vietnamese visas from the embassy. Eventually the new visas were acquired and stamped and we were allowed to proceed.
The other forty passengers were very very unhappy with the two people who had bought the wrong visa and whose use of that visa stopped everyones' holiday in the middle of a river. They had no visa when they should have had easy passage.
The moral of the story is be prepared and have your visas in your passport for every border crossing.