It appears only in certain conditions, after using voice to write messages
The Windows Phone 8 Portico platform release reportedly packs a rather odd messaging bug which appears when trying to add more text to a message after using voice to write the rest of the content.The issue, found to affect at least the Nokia Lumia 920 devices on AT&T’s network, was signaled by Todd 4 Tech, which describes it in detail (via WMPoweruser).
The issue manifests by adding a second recipient to the already chosen one, which is, in fact, the last name of the recipient, without a number attached, which results in failed delivery to it.
What’s more interesting is the fact that the issue appears only when the People Hub on the Windows Phone 8 handset has been set to display contacts name by Last, First. Apparently, all can be easily resolved through switching to First, Last or Name, Surname.