You know how you sometimes make a mistake in one line, but after doing a few lines, you go back to actually writing the equation correctly? Happened to me all the time in uni. It's basically because you were thinking of doing the next line or whatever, and you just forgot that a var or const was somewhere in there, or you just didn't copy (or copy it correctly) in the next line, but the memory of that var/const remained in your brain, so after doing a few lines, the equation is now simple enough so your brain knows something should be there, but it's missing. Sure, we almost always caught up with the mistake, go back, correct the last few lines and carry on. But, every once in a while, you don't, and you carry on solving the equation, and you get a correct solution, but from a purely mathematical standpoint, yes, that solution is not correct.

My math proffesor in uni had an interesting take on this. He said, you didn't do 1 mistake and then correct it to get the right answer, but you actually made 2... which is worse... according to him. And I have to say, at that time, I didn't agree, but let's be honest... he is correct. So, he went a lot harder on those students that did this type of mistake than the ones that just made 1 and carried on solving the equation like nothing happened.