This drives me absolutely crazy! I have to answer email support question, and it completely derails me when writers put "then" rather than "than". Sometimes I have to re-read a sentence a couple of times before I can understand what they're saying. Admittedly I've made the same mistake (typo, though, not grammar), but I always catch it in a re-read before sending an email out.

The only thing worse is when people TYPE IN ALL CAPS. I have no idea why they do this; I can only assume that there’s probably a technical reason why some customers have to write in a single case. It makes their emails very difficult to read, though. Incidentally, I also receive emails in all lower case, and while this bugs me it’s not near as bad as all caps. I do ok with it’s/its: while I always notice it, it doesn’t derail my brain. Of course I don’t have perfect grammar, but there are a few things that just really make text difficult to read.

*sigh* I just wish people would put more time into their emails before they click the “send” button. I do it for them!