Awesome addon, it's been great on our forum. Kudos.
We're having an issue where the entire shop transaction log shows for users that shouldn't see it, including logged out members. This happens both in the shop and on their Activity tab, and is labeled with phrases like "You purchased/received {1} item(s). Click <a href="{2}">here</a> for a listing." It happens randomly - definitely for anonymous users, and then some logged in users. From looking at the product xml it looks like $ownitems[cnt_items] is being counted incorrectly.
I've also been able to confirm this on 3.8.3 and earlier versions. Googling common English phrases from vBExperience will find other boards' shops, and from there you can see the bug in action.
Here are two screenshots of it. The profile one is from a user named Alex, who has never purchased anything. The shop one is from a logged out user (shown with altered phrase).
Profile:
http://img9.imageshack.us/img9/6365/activities.png
Shop:
http://img9.imageshack.us/img9/6602/shopm.png