As a philosophy professor, I have waged a never-ending and largely pointless battle against people using “fallacy” when they mean “factual error.” For example, someone might say “people often think that dogs won’t eat anything that is bad for them, but that is a fallacy.” This is, of course, a mistake. A fallacy is a mistake in reasoning that occurs when the premises presented in an argument fail to adequately support the conclusion.
Naturally enough, people might argue that words should mean whatever people say they mean. So, of folks use “fallacy” in place of “factual error”, then that is just they way things are.
While that has some appeal, since language is mostly a matter of convention, this sort of use is problematic. After all, there is an important distinction between an error in logic and a factual error. It certainly seems important to distinguish between those two mistakes. To see why, think about balancing your checkbook. You can make a mistake by doing the math incorrectly (adding $500 + $50.50 and getting $555) and you can make a mistake by entering the wrong amount (for example, $50 instead of $500) for a check. These errors are different and calling them the same would be a mistake and would also cause confusion.
Now, if people insist that “fallacy” should mean the same as “factual error”, then a new word would be needed to name what we used to call “fallacies.” However, since we already have a perfectly good word for fallacies, namely “fallacies”, then it makes sense to simply stick with the current usage.
A minor problem with using “fallacy” for “factual error” is that it makes teaching logic a bit more challenging. To be specific, it is quite challenging to make the distinction between assessing the quality of reasoning and assessing the quality of a claim without adding to the confusion by using two terms for the same thing.
As such, it would be nice if people would stop using “fallacy” in place of “factual error.” One refers to a mistake in reasoning and the other refers to being wrong about a fact and these are not the same things.