Hi naumira, When someone talks about common error, he is making reference on the errors people tend to make repeatedly, like forgetting to capitalize, using incorrect punctuation, and incocrect spelling etc. among others.
I am traveling this week and don't have access to my files of suggestions. If you will go to my blog at (URL removed by mod)* you can scroll through the entries and find many, many examples of usage goofs in English, along with my suggestions for correcting them. I think you would find this helpful.