It's usually hard to figure out whose in the wrong in these situations.

If someone in the group say "I notice you're using -blah- when you should be using -blah-", the person playing incorrectly would be in the wrong for going "To hell with you! I play like I want!"

On the other hand, if someone in the group says "Omg you're an idiot. Wtf are doing casting -blah-. Are you retarded?", then pretty much anyone would get defensive.

It's all in how you approach it.