Putting a comma before 'or' and 'and' in a list. Sentences which are run-on (too long) Unnecessary commas after 'although' and missing commas after 'however'. Sentence fragments - sentences missing needed subject and/or predicate. Apostrophe errors on possessives - especially plurals.

Another comma mistake is thinking a comma can tie statements together (it actually does the opposite). I see this from both native speakers and learners far too often. Keep commas to a minimum. Then there is... an excessive use... of ellipses... One other modern annoyance is from people who think an emoticon can replace punctuation :(