Here’s the thing: if any of us are in error on a point of doctrine, then objectively speaking we are in fact heretics, not orthodox. Doctrinally we’re in error.
Being doctrinally in error is not necessarily the same thing as committing the sin of heresy, though. (Unless gnosticism, salvation by doctrinal belief, is true. Which is, well… a heresy! 
Insert multiple levels of irony here.)
The sin of heresy could be committed by someone teaching simon-pure orthodoxy, so long as the person is doing so for their own advantage, in order to ‘go their own way’. Abuse of truth is the sin of heresy.
The typical example, however, would be someone taking a (nominally) ‘heretical’ position and being proud of doing so. Chesterton had some chewy things to say about this. But we see it in our day (as he did in his) when an author irresponsibly throws up a book for fun and profit, literally leveraging the faith of other people in order to advance himself. Dan Brown would be the modern archetype of this; the Jesus Tomb guys come to mind as well.
This, by the way, is why I am quickly losing patience with the Rob Bell situation: he’s going out of his way to sully his opponents; but when pressed he doesn’t seem to have solid and deep reasons for doing so; and there’s a huge marketing push from the outset where he stands to benefit from the friction he’s promotionally generating. He needs to get his act in gear.
