Growing up, my family was always correcting my grammar. (e.g. "I have a friend at school that.." and my mom would immediately interrupt with "You have a friend

Growing up, my family was always correcting my grammar. (e.g. "I have a friend at school that.." and my mom would immediately interrupt with "You have a friend who"). I still screw up all the time, but many of these little things like this catch my eye. In every-day speak or in internet comments (Including here on HN), it doesn't bother me at all when the wrong form is used. Casual speak is casual. When I see it in an article, I assume that the author is ostensibly a writing professional; that's when the misuse really irks me. Doubly-so when it's in the opening statements. First impressions and all that....