You'd think this would be a fairly simple concept, but you'd be surprised how often and in how many ways people violate something this basic.
I spent a lot of my life with some exceptionally broken mental models: about money and work on the one hand, and about relationships on the other*. It wasn't until I started adjusting them that I started having more success in either field.
We can't get along without models of reality. Reality is just too complex for our limited brains, and models are a useful construct to allow our brains to make some kind of decision given complex reality. (Jonah Lehrer talked yesterday on Fresh Air about how people are overwhelmed by excessive complexity when trying to make decisions.)
My mental models still have a lot of issues, and I do have some concern that I may run into the problem Taleb keeps talking about, where your model is good enough in the current circumstances but may somehow be prone to abrupt and catastrophic failure when conditions you have assumed not to ever be worth worrying about turn out to be much more common than you expect. Then they blow up. (The model, your relationship, the financial markets, whatever.)
*I'll be glad to discuss these, of course, but this post was just about adjusting models to fit reality rather than rewriting reality to fit an existing model, as opposed to what the model itself is.