Looks like someone else's activities are showing up in my account for the first week of August 2011. Some of them overlap in time, and are clearly from different accounts. For example, the following activities are showing up on Saturday, August 6th:
http://connect.garmin.com/activity/104457113
http://connect.garmin.com/activity/104456648
http://connect.garmin.com/activity/104456242
http://connect.garmin.com/activity/104455377
http://connect.garmin.com/activity/104456660
Only one of those is actually mine (104455377).
Any ideas?