I guess that technically this is an issue of syntax rather than of grammar, but it drives me crazy when people come forth with sentences like "In third grade, my parents got divorced." Of course what the speaker meant was "When I was in third grade ...." We hear more sophisticated versions of this in the broadcast news all the time. We have to infer who did what, and it isn't always as easy as figuring out that his parents were not in third grade at the time.
The word at used at the end of a sentence. When I was in elementary school, and said a sentence with the word "at" as the last word, she would say "behind the at". I would then have to repeat the sentence. Example" Where are we at?" Should be "Where are we?" To this day, whenever I hear someone say "at" as the last word in a statement, I can hear my mother saying "behind the at".
One of the most annoying grammatical errors is the use of "irregardless". I've been teaching in my current school for 20 years and teaching with a lady who's been there 35 years. Several years ago, the school hired a principal who attended HS in this building and this colleague of mine taught him English III and IV. On the first day of staff training, he was speaking to the faculty and used the word "irregardless" to which my colleague promptly corrected him in front of everyone! It's a good thing he was a good sport about it.