You have done nothing wrong. This is a problem that some people are experiencing intermittently.
Try to type in your message rather than using cut-and-paste.
If there is an option below the bottom right of the edit/reply box that says "Switch to Full Reply Screen", select that option.
Also, there might be a drop-down menu at the bottom left of this box. If there is a choice, select "Using UBBCode and HTML"
Good luck,
