Footnote in a footnote.
Invented by Brian Scearce (according to PeterVanDerLinden?, see http://rtfm.rdslink.ro/Books/C/Prentice_Hall-Expert_C_Programming_Deep_Secrets.pdf page 19).
Invented by whom? Well, on a completely unrelated note, I had a chemistry teacher who wrote his own chemistry book (or rather, lecture notes). He was very fond of footnotes, and used asteriks for footnotes. Just a single one (*) for the first one, two (**) for the second one, and so on. The maximum I ever counted was nine (*********), which is quite hard to distinguish from the eight before (********). Anyhow, if in the footnote there was another footnote, he used numbers: (1), (2), (3), and so on in regular footnotes. On the second level (footnotes of footnotes), he used two-digit numbers (11), (12), to nest another level. I remember a footnote like (1324). Needless to say, I (literally) burneded the notes after the course...