What if someone discovered that the specifications in a font file could be Turing complete [wikipedia.org]? What if that person realized that a font could, therefore, perform computations. How about addition? Have these characters:
Proving the Turing Completeness of Fonts [blogspot.com]:
The goal is:
I wanted to try to implement addition. The input glyph stream would be of the form “=1234+5678=” and the shaping process would turn that string into “6912”.
The sheer number of details precludes a simple summary. Mix a little recursion with a strong helping of remapping to implement some grammar productions and voila! The font file is available on Google drive [google.com].
What "creative" [mis]applications of this technology can you think of? Define a font file that has a 1:1 mapping of all ASCII characters... except replace all instances of "123" with "456". How could you recognize this had happened to you?
Consider: imbedding it in a web page or a PDF document. Making it a new (default) printer font.