Introspection and Expression
I've been working (for years, I'm afraid) on an essay called The Unreliability of Naive Introspection. A common reaction to the essay's title is this: What, We don't know what we believe and how we're feeling? That's nuts!
I might be nuts, but I'm not that nuts. I do think that we're fairly good judges of what we think and how we feel. I can still hold the view that our introspective judgments are generally unreliable because I don't think such judgments are grounded in introspection. Instead, I'd call them expressive.
Here's the idea. When someone asks you "What do you think about X?" You don't cast your eye (metaphorically) inward. You don't attend to your experience or think about your mind. Instead, you express what's on your mind. You reflect on X, perhaps, and allow yourself to render aloud your judgment on the matter. This is a very different process from thinking about what your visual experience is right now (e.g., whether it's fuzzy 15-degrees from the center of fixation) or from trying to decide whether your present thought is happening in inner speech and if so whether that inner speech involves auditory imagery, motor imagery, and/or something else. In the latter case, you are attempting to discern something about your ongoing stream of experience. In the former, you're not. My beef is only with the latter sort of judgment.
Wittgenstein famously characterized sentences like "I'm in pain" or "that hurts" as just a complex way of saying "ow!" or grimacing -- in other words, as an expression in the strict sense in which a facial expression is an expression -- a more or less spontaneous manifestation of one's mental state. But even pain we can reflect on introspectively. If the doctor asks exactly what the pain in my finger is like, I can attend to my experience and say "well, it's kind of a dull throbbing in the middle of the knuckle". The difference between introspective judgment and expressive self-ascription is the difference between such reflective descriptions and a spontaneous "that hurts!"
But maybe it's not fair to compare the accuracy of a very general self-ascription ("that hurts") with a rather specific introspection ("shooting pain from here to here"). In the case of pain, I suspect, very general introspections ("there is pain") will tend to be fairly accurate.
However, self-ascriptive expressions of belief, unlike pain, can be pretty specific: "I think that fly will be landing on the ice-cream shortly" -- similarly with desire, intention, and many other propositional attitudes (for a definition of "propositional attitudes", see the second paragraph here). I doubt that I am similarly specifically accurate my self-reflective introspections about what exactly my stream of experience is as I think about that fly.
Emotion commonly lends itself both to spontaneous self-ascription and to reflective introspection. When someone says "I'm depressed" or "I'm angry", it's often hard to know how much this is expression vs. introspection. But in adding detail, people tend either to go expressive, treating the emotion as a propositional attitude ("I'm angry that such-and-such") or to go more strictly introspective ("I'm experiencing my anger as a certain kind of tenseness in the middle of my chest"). It's only the last sort of judgment I would argue to be unreliable.