"login as user" chooses wrong user
In the clients list, there is a button to login as a client (only for the admin). When you click that button, it asks for confirmation whether you want to login as a different user than the one you clicked on, e.g. a reseller. If you confirm, you will indeed be logged in as the wrong user. If you decline, it takes you back to the user list, which now contains all users (not just clients). If you click on the button in this list, it actually selects the right user.
(Probably related to a list index being passed to the backend instead of the username.)