why is my Galaxy S5 sending some of my texts with parts of other texts mixed into the current one?

[AC Question]

So far it has only happened with one person and we are both on AT&T.

 

[Rukbat]

That sounds like a corrupted message database. (I'd use SMS Backup & Restore to save it, fix the resultant .xml file, clear the existing database, then insert the entire .xml file - but that requires knowledge of SQL. If you don't need the current messages, of saving them as text files is enough, use SMS to Text to save them, back up everything else on the phone that you need [see Backing up an Android Device], then do a factory reset.)