Steps:

In JIRA v7.0.0-OD-02-243 (on a dev instance)
Navigated as sysadmin to User Management, selected user A
In another tab, navigate to User Management, selected user B
In user A tab, Clicked Log in as user button. As user A, finished signup and viewed issue.
In user B tab, Clicked Log in as user

Outcome:
Got a nonspecific error message (see screenshot)

Expected:
Ideally replacing my impersonation, or at least an error message indicating the problem