Grammatical errors are easy to overlook but are blatant when someone
is trying to read what you are writing. I've included some other common
situations that, while not grammatical errors, are frustrating for a reader.
TYPING IN ALL CAPITAL LETTERS - This is considered "shouting" and can be very hard to follow your train of thought. If making a POINT, capital letters are generally acceptable. But, DON'T GO OVERBOARD.
Capitalizing The First Letter Of Every Word In A Sentence - I see this often and don't really understand the reasoning unless the writer doesn't know what words should begin with a capital letter so they take the easy way out and begin each word with a capital letter.