A humorous thing to stick at the end of a long list of exclamation marks, mixed in with 1's. n00bs often put lots of exclamation marks at the end of their statement. Sometimes they let go of the shift key partway through, ending up with "!!!111!!" instead of "!!!!!!!!". So people started making fun of that by actually typing out "SHIFTONE".
OMG my brain hurts!!!11!!!11!SHIFTONE!111!