Maybe a list of common errors in English usage would be more useful if it were a manageable list. How about the 20 most common errors in usage. I'd bet (but surely there are some studies on this) that the following would appear near or close to the top of the list:
it's for its; there for their; alright for all right; 'the reason is because', for 'the reason is that'; dangling modifiers; missing or unclear pronominal antecedents; and subject/verb agreement.
it's for its; there for their; alright for all right; 'the reason is because', for 'the reason is that'; dangling modifiers; missing or unclear pronominal antecedents; and subject/verb agreement.