I am guilty of various grammatical errors, but the singular rule that consistently makes me double-check myself is the difference between 'it's' and 'its'.
I'm always inclined to use "it's" as a possessive pronoun and have to remind myself that no, it solely signifies a contraction. i.e. "It's walls are covered with paint" seems like the most normal usage to me! Gah!
I know! I know! This seems an elementary problem, especially when you consider that their/there/they're is a non-issue for me most of the time and I (usually) don't have trouble with other contractions!
This whole handicap reminds me of the days when, long after I could proficiently read and write, my littlest sister was learning how with phonics-oriented curriculum (as I did not). I remember looking over her homework, and questioning my own fluency. After concluding that yes, I could still hablo ingles, I decided that it was a very inadvertent way to acquire literacy.
Silly grammar. I tip my hat to anyone learning English. It is a language that often intimidates me, even though it's (got it!) my first.
What are your grammar hang-ups?