There are three great besetting sins of rationalists in particular, and the third of these is underconfidence.  Michael Vassar regularly accuses me of this sin, which makes him unique among the entire population of the Earth.

But he's actually quite right to worry, and I worry too, and any adept rationalist will probably spend a fair amount of time worying about it.  When subjects know about a bias or are warned about a bias, overcorrection is not unheard of as an experimental result.  That's what makes a lot of cognitive subtasks so troublesome—you know you're biased but you're not sure how much, and you don't know if you're correcting enough—and so perhaps you ought to correct a little more, and then a little more, but is that enough?  Or have you, perhaps, far overshot?  Are you now perhaps worse off than if you hadn't tried any correction?

You contemplate the matter, feeling more and more lost, and the very task of estimation begins to feel increasingly futile...

And when it comes to the particular questions of confidence, overconfidence, and underconfidence—being interpreted now in the broader sense, not just calibrated confidence intervals—then there is a natural tendency to cast overconfidence as the sin of pride, out of that other list which never warned against the improper use of humility or the abuse of doubt.  To place yourself too high—to overreach your proper place—to think too much of yourself—to put yourself forward—to put down your fellows by implicit comparison—and the consequences of humiliation and being cast down, perhaps publicly—are these not loathesome and fearsome things?

To be too modest—seems lighter by comparison; it wouldn't be so humiliating to be called on it publicly, indeed, finding out that you're better than you imagined might come as a warm surprise; and to put yourself down, and others implicitly above, has a positive tinge of niceness about it, it's the sort of thing that Gandalf would do.

So if you have learned a thousand ways that humans fall into error and read a hundred experimental results in which anonymous subjects are humiliated of their overconfidence—heck, even if you've just read a couple of dozen—and you don't know exactly how overconfident you are—then yes, you might genuinely be in danger of nudging yourself a step too far down.