It is book pages that read (as shown below) that:
This page intentionally left blank.
The Urban Dictionary more correctly says:
The rest of this page intentionally left blank.
Four other variations are:
This page intentionally left (almost completely) blank.
This page intentionally left blank,
except for the preceding phrase.
This page has been intentionally left, except for
this annoying little message, entirely blank.
[This page is intended to be blank. Please do not read it.]
Wikipedia has an article that discusses why there are intentionally blank labeled pages.
The Language Log for February 19, 2010 on Language and meta-language talked about the intentionally ‘blank’ page. It has a long set of comments, including about one famous painting by René Magritte called The Treachery of Images which shows a pipe. Below it, Magritte had painted:
"Ceci n'est pas une pipe.", French for "This is not a pipe."
I commented that one of Dave Kellett’s Sheldon cartoons, from February 15, 2010, repeatedly showed Arthur the Duck with captions reading:
“This is not a duck. This is not a duck floating on a lake. This is not a duck noticing us talking about him floating on a lake. This is not a duck who enjoys Magritte jokes.”
The image by Takashi Hososhima of an Oxford University Press bookstore came from Wikimedia Commons.