Retracted my previous comment, because it was agreeing with your claim that it's better to never hold grudges in the first place, which I quickly realized I also disagreed with.
A grudge is an act of retaliation against someone who has harmed you. They hurt you, so you now retract your cooperation - or even engage in active harm against them - until they have made sufficient amends. If they hurt you by accident or it was something minor, then yes, probably better not to hold a grudge. But if they did something sufficiently bad, then it is better to hold a grudge to show them that you will not accept such behavior, and that you will only engage in further cooperation once they have made some sign of being trustworthy. Otherwise you are encouraging them to do it again, since you've shown that they can do it with impunity - and by this you are also harming others, by not punishing untrustworthy people and making it more profitable to be untrustworthy. You do not forgive DefectBot, nor do you avoid developing a grudge in the first place, you hold a grudge against it and will no longer cooperate.
In this context, "forgiveness is a good thing" can be seen as a heuristic that encourages us to err on the side of punishing leniently, because too eager punishment will end up alienating people who would've otherwise been allies, because we tend to overestimate the chance of somebody having done a bad thing on purpose, because holding grudges is psychologically costly, or for some other reason.
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.