Just that moment. I definitely didn't follow any of its implications. (Other than "if I say this then people will react as if I said an obvious true thing.")
In my case such "short term mistakes" are often caused by fatigue. It's as if my brain enters some kind of energy saving mode and sanity checks are deemed not quite as necessary as some other things. In one case I somehow managed not to notice a contradiction in the idea that a cube has four sides and because of that I failed to solve a problem in a school mathematics competition (it must have been one of the problems, as I must have been really tired by then). It seems to me that sanity checks are analogous to redundancy and duplication of compo...
What is something you used to believe, preferably something concrete with direct or implied predictions, that you now know was dead wrong. Was your belief rational given what you knew and could know back then, or was it irrational, and why?
Edit: I feel like some of these are getting a bit glib and political. Please try to explain what false assumptions or biases were underlying your beliefs - be introspective - this is LW after all.