I don't believe people generally say things that they don't mean when they're angry. I think when someone is frustrated or pissed off, what comes out of their mouth is most likely how they truly feel.

What's your take on "I didn't mean it, I was just upset?"

It depend on how evil u are I guess. If somebody the type that LIKE pushing buttons they might go out their way to say something that aint true for the sake of just being evil but if you not then yeah the more frustrated u get, the less you thinking about trying to be tactful. From there u just gonna throw out what u really think cause you mad and not thinking about being nice or considerate.

For me personally, I will say things in anger that I really do mean, but wouldn't have said otherwise. But I know for sure that many people do say things that are way outside how they really feel when they are mad. Like Wads said, it depends on how evil they are. People like that really do suck.
What came to mind immediately upon thinking about this is a few years ago, I was on the board of directors for an organization. Someone also on the board of the organization was very thrown and mad at my resignation, and she actually said to me that there were several things that didn't "add up" in regard to how I handled the finances. She said it to me and to others and that is a terrible accusation when it is 100% untrue and unfounded. And she said it because she was angry and she knew what she was doing.

I agree that how someone instinctively reacts to a frustrating situation, or when they are angered, is a testament to what kind of a person they actually are...and if they spout off at the mouth, what they say is most likely what they had been thinking about all along. For me, I try to exercise tact and consideration, and will hold my tongue about something if that something is petty. However, when I get pissed off, I will go ahead and unleash everything I ever thought about you that I shrugged off. I will tell you about how loud your car is, how ugly your dog is, how much I don't like your uppity sister, and remind you that you didn't repay that $5 you borrowed two months ago.

It's totally possible to say things in anger that you don't mean, esp if you know it will piss the other person off. And being angry gives you the excuse to go off and express how you really feel. So it goes both ways. You just have hope that whatever you say doesn't come back to bite you in the ass. People can and will hold what you say against you even if you didn't mean it.

I think things said in anger are things that have been festering inside of you for a very long time and finally when you get to the point of letting is all go, it all comes out. Things sound worse when they come out in an angry battle with someone. I can be a hot headed Italian so I try to keep my anger in check when having an argument with someone and try as best as I can not to hurt their feelings. I would rather bring up the subject right away then to have it brewing inside of me and then lash out in anger and hurt someones feelings.

I agree with Tony that when we're angry, things that weren't even that serious are likely to come out and sound more explosive than they are. So in a way it's a combination of both. I noticed this thing about you before but didn't care that much, but since I'm mad at you now it's an issue that I'm gonna act like has more significance than it does.