I feel I am reasonably competent when it comes to using correct grammar and punctuation, but I am definitely conscious that there are rules I don't know. Chances are I learned them at some point and just forgot. Two examples came up just last night and it occurred to me: I lost the ability to absorb any new grammar rules in the 10th grade. I remember the day it happened, too. I was sitting behind Patrick Abernethy in Mrs. Tomes' first period English class. We were in the middle of diagramming sentences and suddenly I couldn't remember what the fuck a past participle was. And I knew I would never know again.
(I do recall that one is not supposed to end a sentence with a preposition or begin a sentence with "and" but those are often inconvenient for me.)