I have a Juniper EX4200 series 8PoE that was donated to me as a lab asset. The problem is tha it has a configuration on it and i have no usernames/passwords. When i go through the password recovery process this is what happens:
You can use single user mode from the console port to do a root password recovery following this kb article. This does require you power off the switch and have physical access to the serial console port for the operation.