Dungeon's Master has a good post on common 5E mistakes. Many of these are related to 'conversion' mistakes, changes 5E makes from previous versions but maybe we are still using the old version rule.
The post can be found here. Or here is the full URL - http://dungeonsmaster.com/2016/04/10-mistakes/
I did pretty good, I only missed the spell ready part of #8. There is one area that I cannot confirm, it is the >20 on the second bullet of #10. I have never heard of that.
Otherwise, I nice list and useful for your players.