Double negatives: "I'm not doing nothing wrong!"
Not understanding plurals: "John and Fred is friends," which occurs unfortunately frequently in longer sentences. Or, the worst, "The data shows..." "The oxen is..." etc.
Mixing British and American spelling. Choose one and stick with it. it doesn't matter which. Just be consistent. You can't mix-and-match.
And homophones. Especially those with apostrophes. "It's" and "its", "who's" and "whose", "they're" and "their" or "there", and so on. They're not hard. You only have to learn one very simple rule for them and that's it. I teach five year old children how to do this and they manage fine.