They mentioned the Droste effect, but not the fact that by Brouwer fixed-point theorem[0], there is a fixed-point, a point that every image contains.
Similar with mirrors, or cameras that capture their own images (although there is a delay (there is a delay with mirrors too)), or in the cases of maps. Did you know that if there is a map of an area in the depicted area, then there is a fixed point, which is in the case of maps a 'You are Here' point, that is, it is above its location?
Maps of Chicago typically don't have a way to include lower Wacker, and plenty of people haven't even heard of lower lower Wacker.
Even Google Maps struggles with this, bouncing between upper and lower on street view, and pickups with Uber and Lyft are iffy on that street (at least back when I first installed them, gave up on it after a while).
Not so alarming if the map comes equipped with a suitable adjoint (best approximation to inverse) map with which it may be composed; it's by pulling back that "taste classifies, and it classifies those who classify."
If I really wanted to look up the discussion of recursion or infinite loops in a book index, I would be rather annoyed if a cute joke would cause the actual reference to be absent.
In my 1990 copy of Steele's Common Lisp The Language (2nd edition), the index definitions of iteration and recursion cross reference each other, but do point at the actual definitions as well.
However, the joke that he does use in the index is that the entries for the Teenage Mutant Ninja Turtles (who are used an example of a list somewhere) also includes the dates of the renaissance artists (Michelangelo, etc) they were named after.
I'm not sure which is the more worrying: that I noticed this in the first place, found it really funny, or that I remembered it 30 years later.
Similar with mirrors, or cameras that capture their own images (although there is a delay (there is a delay with mirrors too)), or in the cases of maps. Did you know that if there is a map of an area in the depicted area, then there is a fixed point, which is in the case of maps a 'You are Here' point, that is, it is above its location?
[0] : https://en.wikipedia.org/wiki/Brouwer_fixed-point_theorem#Il...