I had similar on a VAX 11/750. The LA120 DECwriter was connected to the system via a serial port mux adapter.
One night the mux adapter glitched and reset itself back to defaults, which turned on local echo, so it reflected 'login:' back to the VAX, which responded with the password prompt.
This prompt was reflected too, giving an incorrect password response...which was reflected back to a new login prompt...etc..rinse, repeat all night.
Everything was printed on the LA120 until the fanfold paper ran out.
I came in the next day and could not see the LA120 due to the amount of paper on top of it.
One night the mux adapter glitched and reset itself back to defaults, which turned on local echo, so it reflected 'login:' back to the VAX, which responded with the password prompt.
This prompt was reflected too, giving an incorrect password response...which was reflected back to a new login prompt...etc..rinse, repeat all night.
Everything was printed on the LA120 until the fanfold paper ran out.
I came in the next day and could not see the LA120 due to the amount of paper on top of it.