I'm only sort of kidding. I spent two years teaching English, but in a much, much different setting--at a boarding high school for gifted math and science students in Gongju, South Korea. In the casual setting of the bar, I encouraged these (adult) students to ask questions, to press me on the reasons for any changes they didn't intuitively understand, and to talk about their writing choices.
One student, after hearing me cite the rule behind a particular common error, asked tentatively, "Is there anything you still have to look up when you're editing?"
I almost snorfed my lovely-yet-affordable Shiraz. "I look stuff up all the time!" I told her. "I have the Chicago Manual of Style Online and Merriam Webster set to open tabs when I start my browser."
The truth is that even the most experienced editors have to double-check stuff. Every writer and editor I know has at least a few bugaboos, rules or distinctions that somehow just seem less intuitive than the rest.
So in November, I started keeping track of what I had to look up--and publicly owning up to each one. Being a good proofreader or copyeditor isn't about knowing all of the rules cold; it's about figuring out where your blind spots are and filling in the gaps with expertise, either by consulting the right resources or training yourself to understand and remember something you've struggled with.
On Twitter, you can find the things I've had to look up recently under the hashtag #TIHLU, for Things I Had to Look Up. The first five are below--and you're welcome to Tweet your own!