Meditations, Book I, Section 7

Meditations, Book I, Section 7:

From Alexander the Grammarian, to be un-reprovable myself, and not reproachfully to reprehend any man for a barbarism, or a solecism, or any false pronunciation, but dextrously by way of answer, or testimony, or confirmation of the same matter (taking no notice of the word) to utter it as it should have been spoken; or by some other such close and indirect admonition, handsomely and civilly to tell him of it.

I think Marcus is talking here strictly about language usage, not ideas conveyed by language. The dictionary says a solecism is a 'grammatical error' or a 'breach of good manners'. In 2014, with auto-correct on many platforms, you encounter language errors less often than ever before. 

I would argue that if you see another person's language error, it's often good form to mirror their error for the duration of the conversation, so they aren't made to feel inferior by way of indirect correction.