Is it possible that after a messenger update, a text from my blackberry to someone's phone could be completely mixed up with someone else's in the process?
Are you talking about messenger (like MSN, Yahoo) versus SMS? I'm not sure if that is possible; but I don't use messenger on my phone often enough to know. But they're two separate functions - messenger and SMS - so I don't know if they'd be interconnected like that. There might've been a bit of a lag due to the messenger update and then an SMS going out; but I still don't know. Is the issue still occurring? Maybe uninstall and re-install messenger?
Hmm. If that ever happens again, contact your carrier. The different text your friend got - was it a text you had written at some point to someone else that they received; or had you never written it before? If you're concerned, you could go to your carrier's store and ask why it happened; or sent a direct message to @blackberryhelp on Twitter.