I'm guilty of saying "Which do you like best?" when comparing two objects, when I should technically be saying "Which do you like better?" or perhaps better yet, "Which do you prefer?"
I often confuse when I should be capitalizing the first letters of Dad/dad, Mother/mother, etc.
I don't know if til is a word. I know 'until' is a word, and 'till' probably is, but not so sure about til. On a related note, when people say the time is "quarter til eight" I understand that they mean 7:45, but when they say "quarter of eight" I am not sure if this means 7:45 or 8:15.
I over-use hyphens!