Quite often on a course, especially one that goes through farmland or has high barbed wire fences, it will be compulsory to cross at a certain place, rather than being able to go wherever you like. If you cross elsewhere, officially you can be disqualified.
Your control descriptions tell you if you have a compulsory crossing point as shown below. (It’s the one between the start and control 1)
Here’s an example on a map – as you can see, the pink line between controls will bend to go through the crossing point sometimes; however this is not always the case. 
In the example above, taken from last Saturday’s race at Downhill, the Green, Blue and Brown courses all had this compulsory exit from the start control. If, for example, you did this…
…you could be disqualified. Instead, once you had looked at your map, even if you had already reached the un-crossable wall, you should have gone this way…
This gets you out of the start much faster than the other routes, and is actually legal, where route 1 is not.
The reason I am posting this now, for those who are interested, is that I stupidly took route 2 on Saturday, because I didn’t look at the map soon enough properly. The control descriptions did tell me, but they were folded to fit in my description holder so I didn’t see the compulsory crossing point symbol.
Paul Pruzina 14/6/2013