Not understanding what an army can do the first time you face can be frustrating and devastating. Afterwards its always the "I should of done this, or I should of done that". Knowing your opposition articles are just that, to help stop the surprise and to help you (and me) to be prepared for what is about to happen on the tabletop.
An scout army is quite rare, however there are a lot of players lists that have some form of a scout move in them. For now we are going to save the outflanking for another article, and just briefly talk about what a scout move can do, and what you can do to stop it.
Imagine this. You set up your dual Land Raiders with your termies ready to go on the front line. Your opponent is a guard player with some veteran units, a couple vendettas or Valkyries, and a bunch of back line tanks. Before the game even starts, he moves his vendettas forward during his scout move. They move 24" putting themselves just outside of 12 inches from your Land Raiders. 12.1"'s to be exact. With his scout move done, and you failing to seize the initiative, its now his first turn. He starts off by deploying out of the vendetta, a unit of 10 veteran guardsmen with the demolitions doctrine. They deploy 2" move 6". Then his assault turn comes, and he assaults another 6" assaulting both Land Raiders with a total of 10 melta bombs. Worse he just did that with a single 100pt unit.
Scout moves are a pregame move, that you must stay outside of 12" from your opponent. They are very very useful for delivering crippling strikes to the enemy round one. Vendettas, Baal Predators (with flamestorm cannons), scout bikers, and many other units can do this effectively.
So how do you stop this from occurring. There are many ways, first is to just screen those land raiders or other units. Simply putting the terminators that are inside the Land Raider in front of it, will stop the assault in the above example. Putting your tanks or units slightly back, or screening them with units not really effected by the scout units. Also making it difficult for the scout units to move in is a great way to hamper their movement. This can be done with infiltrators, or other units that will limit how close the scout units can get close to their optimum target.
The primary thing is to not let the game basically be over before it really begins when your opponent has a scout unit. The above example with the vendetta, is one of the tools in my arsenal for my imperial guard (Faeit 212) army, and although Ive not yet been able to assault two land raiders at once, I have gotten a Land Raider and a vindicator on the first turn of the game.