Implicitly assuming that you mapped out/classified all possible realities. One of the symptoms is when someone writes "there are only two (or three or four...) possibilities/alternatives..." instead of "The most likely/only options I could think of are..." This does not always work even in math (e.g. the statement "a theorem can be either true or false" used to be thought of as self-evidently true), and it is even less reliable in a less rigorous setting.
In other words, there is always at least one more option than you have listed! (This statement itself is, of course, also subject to the same law of flawed classification.)
There's a Discordian catma to the effect that if you think there are only two possibilities — X, and Y — then there are actually Five possibilities: X, Y, both X and Y, neither X nor Y, and something you haven't thought of.
We recently established a successful Useful Concepts Repository. It got me thinking about all the useless or actively harmful concepts I had carried around for in some cases most of my life before seeing them for what they were. Then it occurred to me that I probably still have some poisonous concepts lurking in my mind, and I thought creating this thread might be one way to discover what they are.
I'll start us off with one simple example: The Bohr model of the atom as it is taught in school is a dangerous thing to keep in your head for too long. I graduated from high school believing that it was basically a correct physical representation of atoms. (And I went to a *good* high school.) Some may say that the Bohr model serves a useful role as a lie-to-children to bridge understanding to the true physics, but if so, why do so many adults still think atoms look like concentric circular orbits of electrons around a nucleus?
There's one hallmark of truly bad concepts: they actively work against correct induction. Thinking in terms of the Bohr model actively prevents you from understanding molecular bonding and, really, everything about how an atom can serve as a functional piece of a real thing like a protein or a diamond.
Bad concepts don't have to be scientific. Religion is held to be a pretty harmful concept around here. There are certain political theories which might qualify, except I expect that one man's harmful political concept is another man's core value system, so as usual we should probably stay away from politics. But I welcome input as fuzzy as common folk advice you receive that turned out to be really costly.