There isn't just one common grammar mistake - there are many. Did you mean to ask 'What is the MOST common grammar mistake in English?'
If so, the commonest mistake among learners is missing the 's' off the the end of the verb for he, she and it in the present simple : for example 'He speak' instead of 'He speaks'.
The category of mistake you see here on italki is a total ignorance/total disregard of the rules for asking a simple question.