It seems to me, learning the rules of 40k seems to be one of the biggest learning curves of almost any game out there. It's almost unbearable seeing a game played where someone is so obviously breaking the rules, that I don't say anything anymore. It often leads to far too many arguments in games that I really don't care enough about. Despite knowing this, sometimes I can't help myself and will put in a very quick and discreet, "you didn't do this correct, if you want to look it up, its on page such and such". A buddy of mine recently moved, and was checking out the local scene. Apparently their so-called top player was very much in the wrong on how he was assaulting. Multiple assaults apparently don't need to stay in coherency in that town, and if they do, you can move as little as you want in order to maintain that coherency in order to assault in two complete different directions.
So where does all this come from? It is after all a pretty simple rule set, with the least amount of grey areas that I can remember in an edition of warhammer of 40k. Even so, 5th edition is somehow ridden with more misinterpretations and misreading that I have seen in previous editions. Number one problem, silly Internet interpretations. Not to smack dakka dakka, but their forums are full of some pretty extremely silly Internet rulings like obscurement (see here if you are in that crowd http://natfka.blogspot.com/2010/05/obscurement-much-to-your-demise.html). Second problem, mathhammer. Math hammer tends to lead to sometimes creative min/maxing, and the rules sometimes get lost or overlooked at this point. It's not that math hammer is bad, knowing the probable chances of something happening can explain why your single lascannon shot cant kill a rhino. Some people just get carried away with min/maxing. Back in 3rd ed, it was all about it, with 5 man squads and heavy weapons. Last problem, is bad advice. Know who you are listening to, and look it up..... please.
While these reasons are valid, they often lead to the most disputes. If someone just doesn't know the rules well, or makes a mistake, its often very very simple to correct, and the game moves on. A mistake is a mistake, who cares, and move on. I am on purpose leaving out comments on the just plain dumb, or the double dumb. There is no need to expand on that.
Just remember, that when you come to the table, and both people are playing by a different set of rules, it makes the game hard, and sometimes frustrating. I hate rules lawyering, but I also dislike long drawn out dumb arguments about a rule that is pretty damn simple.