A Piano I Play By Ear: Grammar Rules (an open book blog hop post)

Welcome to Open Book Blog Hop. You can find us every Monday talking about the writing life. I hope you'll check out all the posts: you'll find the links at the bottom of this post.
What grammar rules have you broken on purpose?_______________________________image source
I don't set out to break grammar rules in my writing, but I also believe in using the right words for what I'm doing. Here, on my blog, for example, I strive for a breezy, friendly, conversational style. Sentence fragments, idiosyncratic spacing, and slangy vocabulary are common.
When I'm writing something formal, like a letter to a lawyer or documentation for school or work, I'm much more careful, avoiding even contractions lest my message be misconstrued.
In my fiction, the main narrative is generally grammatical, but there are some things that I was taught as rules that I don't consider that important. Things like: never start a sentence with and or but, don't split infinitives, don't end a sentence with a preposition. How the language feels is more important to me than the strictest of grammatical interpretations.
I worry about genre expectations and tone and voice appropriateness more than all of that. Plus, it makes a difference what point of view I'm writing in. A first person narrative must be "spoken" the way that character would talk, which may or may not be traditionally correct.
That said, I'm a big believer in professional editing and proofreading. I don't want a reader distracted by small errors to the point that they are pulled out of the story! My work has been through peer editing in my critique group before it goes to the publisher, then it goes through several rounds of editing and proofreading once its in the publisher's hands, too.

Some errors always seem to still make it through, which I guess is proof it was worked on by humans and not AI generated :-)
Are there grammar rules you choose not to follow? Or ones that really get under your skin when others don't follow them? I'd love to hear your thoughts in the comments!
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