When you're logged in to Koha with IndependantBranches turned on you shouldn't be able to edit other people's items, but there is a way around this. Even though you don't see the edit and delete links next to the items on this page: /cgi-bin/koha/cataloguing/additem.pl?biblionumber=509 you can click on an item and the edit and delete links show up - once that happens you can click edit and when you go to edit the item it forces the branch to be changed to yours - all bad things.
I have a video, but jing isn't letting me upload it right now, so I'll add it if necessary.
Still bugguy in 3.6.0, i just checked
*** Bug 5289 has been marked as a duplicate of this bug. ***
This is fixed in master. Is it still a problem is 3.6?
Looks like this has been fixed on bug 10225.
*** This bug has been marked as a duplicate of bug 10225 ***