Long time since I fiddled with this, but...

Did you type "ROM" or "rom"? - in Unix case matters and the command is 'rom'...

rom - short for read only mount (counterpart to rwm - read write mount) which remounts the partitions as read only.
Not doing so isn't harmful as such, but an unexpected shutdown (IIRC anything besides explicitly using the 'shutdown' command) should
trigger an fack (file system check) on next startup as the system would be uncertain as to the consistency of the file systems on partitions
that were writable at the time of shutdown.