Unfortunately, there are many common grammar and usage errors in English. This BBC article explores twenty such errors, while Paul Brians' book and website list many more.
What are your particular grammar and usage peeves? Personally, I strongly dislike the incorrect usage of apostrophes and I also don't like when "a lot" is written as one word. But there are many more that annoy me -- and I see them very often in my line of work.
3 hours ago