One error that I encounter often online that makes me twitch slightly is lose/loose confusion, although I would never comment to correct someone. Is there a grammar rule that drives you nuts or that you commit regularly?
I am not very good at remembering many grammar rules myself and probably drive some folks nuts with my dangling participles.{that kinda sounds dirty}I am sure if I had been 'applying*' myself in high school, I would be able to avoid simple errors. Anyways, I like these little illustrations from one of my vintage books:

{* I did apply myself in high school, it just wasn't to school, homework or lessons. A parent's dream.}