Called ‘Antigonish’ and usually referred to as ‘I met a man who wasn’t there’ is a poem written by William Hughes Mearns in 1899. One of the few I recall verbatim from learning as a child.
Yesterday, upon the stair,
I met a man who wasn't there
He wasn't there again today
I wish, I wish he'd go away...
When I came home last night at three
The man was waiting there for me
But when I looked around the hall
I couldn't see him there at all!
Go away, go away, don't you come back any more!
Go away, go away, and please don't slam the door... (slam!)
Last night I saw upon the stair
A little man who wasn't there
He wasn't there again today
Oh, how I wish he'd go away...