I've actually had this happen twice. The first time it happened I texted "I love you" to my wife...and my ex-wife got the message. A-W-K-W-A-R-D
The second time I almost texted a far less appropriate message that was intended for my wife to my son's Cub Scouts den mother. Fortunately I noticed at the last second that my wife's number wasn't at the top of the dialogue box.
For the record, in both cases, the ex-wife's most recent message and the den mother's most recent message were no where near the most recent message from my wife in the text message list, so I have no idea why that happened...
I've learned to be careful before hitting send...