I allow for certain grammatical errors on the computer because when typing quickly everyone doesn't review, but when your and you're, lets and let's, a and an are mixed up by someone who should know the English language, it's a huge turnoff. I remember learning about contractions, appositives, prepositions and other common grammatical errors at a very early age. It just seems that people don't know and don't care anymore.
These gets me upset:
"Supposably" has been added to some dictionaries. Up until a couple years ago "supposedly" was the only right way. My ears burn when a well-educated person says it the wrong way.