My favorite MOTD is:
You might have mail.
I told one of my student operators that if he put that in there before
the morning was out, someone would be in my office to tell me they got
the message but they didn’t have any mail.
I wasn’t expecting it to be one of my senior programmers. Well, it
only said you MIGHT have mail.
Somewhere there was a post circulating around with humorous MOTDs.
That was on it, I believe.