A lot of times when we game we have a tendency to assign the negative traits of a person's character, or the inspiration behind the character's actions, onto the player themselves. If Lothar the Barbarian is a loud mouthed womanizing twit, and you only see John at game, you may start to think that John and Lothar share a lot more traits than just being present in your life at the gaming table. It's simple human nature, but it is a bad aspect of human nature.
In the example above, for all we know, John could be a great person and very quiet. He could be playing Lothar specifically because he is so quiet that he wants to see what it'd be like to be loud mouthed and arrogant. By assuming that John is like his character you do him a disservice, you also deny yourself seeing what could be a very remarkable person.
So, why do I say this? Well, because myself and some friends spent a long time discussing this phenomenon recently, and it has come up fairly regularly previously in conversation. How many times have you been flustered with someone you game with only to later realize that not only is it nothing that it isn't even them you're upset with? It can happen at almost any time. Especially if you're not careful about it.
Next time you see yourself flustered at the actions a character is taking, take a second and make sure that that frustration isn't escaping the game. Enjoy the game for what it is. React to things in the game because that's where they belong. Leave the player out of it, and if their character truly does bother you...well, then talk to them about it. Maybe what you want to do IC to them is exactly what they're looking for to happen at some point.