iMessage sending to wrong person

Occasionally my iMessage will send my text to the person previous in my text inbox. For example, I was texting my boyfriend (with a 3GS) and when I sent the message, I watched it change the recipient to the previous person who had texted me (a day or so earlier), also with an iPhone. The other day I was again replying to my boyfriend's iMessage, but had another iMessage from someone else that I hadn't even opened and it sent it to that person instead. It has happened a few other times, but these two I know I had open the right message thread. So far, I've only noticed it happen with iMessage, not regular texts.

Has anyone ever had this happen? Any thoughts or suggestions on how to deal with it?

Thanks.