Of course these circumstances happen. Over all I am pretty friendly at the gaming table with my opponent, and the people that are cycling up to talk to me. This happened last week, so I figured it was time to throw it out into the open.
A player I didn't know at a local watering hole, was watching the game. I normally do not hit the spot on league night, but I was looking talk to a few players that I do not normally bump into. I don't know this guy, but he seemed alright, and was simply watching the game. I had noticed him earlier, because it was someone I hadn't seen before, and while not impossible, it doesn't happen that often. I was in a fun game, so light occasional talk from the sidelines is OK with me, although I always have to seem to remind people that after the game I would get a chance to discuss what I was being asked.
Here are a couple things to not do at someone else's table during or immediately after the game. It is OK to quickly bring up a rules question about the game. It is not OK to argue it. It wasn't so much that he asked a question, it is that he was trying to correct us on how "feel no pain" works incorrectly.
Being the person I am at the table, I handed him my rule book. It is highlighted and marked up well, so it takes me literally a second between dice rolls to flip to the correct page, and hand it to him. His argument was this for the curious, "A bolter shooting a dark eldar warrior with feel no pain, cannot take a feel no pain roll, because the bolter is AP5, and would never allow the dark eldar warrior to take an armour save since a dark eldars armour save is 5+."
Now being in the middle of the game, I gave a simple "it doesn't work that way", and gave him a quick why. It went back and forth a couple times.
His response "but you could argue that it does because that is what the rule says....".
Which I quickly and sharply cut off before the went into the long reading of the rulebook style of arguing said, "you can argue anything you want, but not during my game." I hate being dismissive towards other players in the hobby.
To make a long post short, he did drop it, and left the table. I felt bad afterwards, because that is not the kind of impression I like to give people. However his table etiquette was definitely off.
The Point: Trying to argue with a person during their game, or immediately afterwards is a definite no no. Remember that you are interrupting the game, so be polite when you talk to a person in the middle of the game, and be brief.