I've been playing table top war games for about 10 years now, well with a little break in college that I'm not fully making up for. And I've noticed a couple things through those years and games and how bent out of shape some people can get over the littlest things. The easiest of these things for people to fight about are just the simple roll of the dice, and not just any die, but a "Cocked One"
Now in general I'm a pretty casual player, who's there to have fun. This is usually much more clear when I play in a tournament, vs the casual games with my friends. It seems that we as gamers or at least the ones I know are much nice and understanding to those we meet across the table in a gaming event, with very little standing in the way of a fun game. But there are instances where that die will be rolled and one can clearly see what the number is and your opponent will say Cocked and want to re-roll it. It makes me laugh to myself, generally I'll let it slide but you know that competitive edge will get to you and you'll raise your voice slightly and say, clearly it was a 3, and you missed your shot.
And depending on your opponent he'll either agree with you and continue playing or he takes his defense stance and begins to inform you as at his house any dice (that misses) and isn't 100% on the table is cocked and must be re-rolled. Depending on how the game is going I usually agree and then point out the hitting cocked die to him as well :D
But it's the games between friends that seem to get the most heated. Maybe because my friends and I are all competitive players, and have always been that way, and maybe it's because we've been friends for so long that any lines of etiquette have long been crossed. It during these games that you'll see emotions run high and rules pulled out of the book every two minutes, that Win At All Cost attitude pouring from the gamer sweat, haha.
I think the point here is that 40k even though competitive is still just a game and as much as I hate to admit it, life does go on afterwards. If we treated everyone with the respect we do when we're trying to receive full credit for our Sportmanship Scores then I think every game that we have will be the best game played. Even when being wiped off the table I've had plenty of fun because my opponent was nice and we made it into a fun time.
Don't get to bolstered up over a single die roll, or that one rule the other player may or may not have know, treat it as a learning experience and remember the dice always change.