Grammarly recently posted this inforgraph of 21 frequently ignored (or unknown) grammar rules and writing mistakes that everyone who writes should know. 

How many do you know? How many do you ignore?

I'm with them on all but the serial comma. I only use it for clarity in my writing. 

<a href="http://www.grammarcheck.net/grammar-rules/"><img src="http://cdn.grammarcheck.net/grammar-rules-infographic.jpg" alt="Bye Grammar Mistakes! 21 Rules to Remember (Infographic)" /></a><br />Source: <a href="http://www.grammarcheck.net/grammar-rules/">www.grammarcheck.net</a>