What are some writing errors you see that fill you with an unreasoning rage that can only be appeased with sacrificing the writer to Baal?
One that annoys me is "let alone." People use it wrong all the time. You'll see something like, "He can't run, let alone walk." And that's wrong. It's in the wrong order. It's little thing, then big thing. "You can't write a proper sentence, let alone an entire novel."
That mistake will also sometimes be used with "never mind" instead of "let alone."
Another one that bugs me is "lead," as in the verb. Specifically, the past tense of the verb "to lead." The past tense of "lead" is not "lead." It's "led." "He led his force to victory." I find this mistake especially irritating, because I regularly see it even in professional publications. Professional writers - people whose job is to know proper grammar - will make this mistake. And editors - whose job is to know even more about grammar than writers - will overlook it.
I'm actually willing to overlook a lot of grammar errors, especially in amateur stories. But those ones make me want to commit murder.
My ex always used to say "on accident" and "by purpose." It would annoy me. Apparently, though, it's a French thing, since her current boyfriend does the same thing. Still annoys me.