1. Log in as an admin
2. Impersonate any other user
3. Drop impersonation
4. Press Login As again

Result:
Error message telling be I cannot log in (bad request).

Note:
Seems to work fine on both Chrome and FF, but reproducible on both IE 11 and Edge.

Workaround
Either temporarily use a different browser, or you can clear the cookies for your current session and then log back in and repeat the operation