As stated in the previous entries by my peers, the Fundamental Attribution Error is the all-too-common tendency to assume the motives or interests behind the actions of another personshape. At its core, FAE is an issue of projection. Our internal state informs our view of another individual's actions.
For example, Humanshape Q might've been recently subjected to poor customer service at the returns desk of a local retail establishment. Feeling peeved and a little peckish, they might choose to stop at a dependable chain restaurant with a drive-thru, where they intend to purchase a favored comestible that will surely improve their sour demeanor. Upon placing their order, Q may be informed in the most pleasant and professional of tones that such an item has been removed from the menu to make room for something new, perhaps a small cardboard carton of rolled flour tortillas containing meat-flavored cornmeal. Due to Q's current emotional state (irritation, for the sake of this example), they might immediately perceive that the employee's tone was snippy and place the blame on them, projecting that said employee is a total jerk who is withholding their order for arbitrary reasons, instead of rationalizing that such a decision was most likely made by panel of sales executives after a battery of taste-tests in a company laboratory. Of course, a few hours later, when Q has had time to let their mood settle, they will realize that such a notion is utterly ridiculous and that the employee was just doing their job (or maybe Q will continue to seethe and hold a grudge, but that'd be another psychological issue entirely).
In this example, Humanshape Q was already annoyed by previous instances and thus temporarily conditioned to assume that other people are unreasonable jackasses. In different circumstances, perhaps in the event that the previous customer service experience had been pleasant, our protagonist Q might've thought nothing of the employee's tone and proceeded to order something else.
It is indeed true that the FAE can be ugly, and most people readily admit to succumbing to it, but it is indeed common and quite usual. The best anyone can do, really, is be aware of it so as to identify it as it manifests. This won't stop it from happening entirely, as humans are so very predisposed, but one can at least nod knowingly and say to themselves, "I see what you did there, brain."