Multiple Android users are reporting issues with the native messaging app on their smartphones, particularly on the Google Nexus One, whereby text and MMS messages are sent to a person other than was initially intended. The problem - which has apparently affected both modified and stock handsets - occurs when the messaging app switches the contact in the To: field of its own accord; sometimes it can be seen to change it, while at other times it happens without any visible change.
Owners of the HTC Incredible have also reported the problem occurring on their phone, so it's not something limited to the Nexus One. Some users have been able to reproduce the fault by maintaining two separate SMS conversations and switching between them mid-typing. Google are apparently looking into it, but there's no official workaround beyond deleting old conversation threads when you're finished with them.