If there is corruption somewhere in the link (example: a bit changed in the serial stream: RF?) you would get that? If you use a higher level hardware or any protocol that sends things in packets, and a packet gets dropped you would get that?
If your code is otherwise working, I would be tempted to move on. Perhaps it will become more obvious.
Of course you can debug it, find where that string is generated and add some more print statements or run it in a debugger.