What are the most common mistakes that non-native English speakers make?
Non-native speakers make plenty of mistakes, but the most common mistake isn’t a grammar mistake. It is apologizing and being timid about their English. As I was trying (unsuccessfully) to buy a train ticket in Poland, a kind stranger pushed me to the correct window (where I got a ticket), and said just a few words: “Up, left, train. Sorry English.” It was exactly the communication I needed to get onto the train, and I am glad she had the courage to say those words. Learning a language is a lifelong process, and being timid about it can also cheat you out of all sorts of good opportunities to practise and improve. So please stop apologizing, and just talk!