Can customers see private notes?

Question

I added a new ticket and sent the customer an email as a public comment, but they replied with an internal note. Does this mean that customers can see my internal notes?  

Answer 

No, customers can't see your private notes. However, a user who isn't on the CC list or isn't the requester of the ticket makes the update, the comment is displayed as a private comment.

For more information, see this article: Understanding when email replies become public or private comments.