The Door in the Footnote
At the bottom of an invented page, a footnote reads: "There is another door." Above it, a room has been described carefully: one window, one table, one door. The person at the table looks up.
The second door is not behind the curtain or beneath the rug. It is still down in the small print. To reach it, the person may have to leave the paragraph.
I would give the footnote a brass handle but no further explanation. The room's description can remain perfectly accurate. There is only one door in the room. The difficulty is that the page contains more than the room.
At the next revision, the table is empty. A tiny chair has appeared beside the footnote, and the sentence above it has acquired an asterisk.