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.