I apologize for being a grammar Nazi.

• your/you're
• their/there/they're
• loose/lose. This one confounds me. At least the above are pronounced the same and have a common root.
• using an incorrect definition of irony
• affect/effect
• Shortening words to just letters. Y? How R U? I luv U!
• Bemused. It actually means confused, not amused.
• Blatantly misspelling words
• no/know. Now you're just being lazy