Here's mine: in black letter printing, there are two forms of the letter "r" (miniscule).
It's easier to show than to describe, so here goes:
Here's what it says: Institutions or princypal grounds of the lawes and statues of Englande, newly and very truelye corrected and amended, with many new and goodly additions, very profytable for all sorts of people to know, lately augmented and imprinted.
Compare the word "princypal" and "groundes" (second and third lines). See how the "r" in princypal looks like a "2," but the "r" in groundes looks like a more usual r?
How look at "corrected" in the fifth line. See how the first "r" looks like a 2 and the second one looks like a more usual "r"?
The "rule" for when one used a "2 r" was whenever it followed one of the letters in "whipboys." That's it. Some trivia I learned in a grad class or something, that has stuck with me for years because it's mildly suggestive. Or something.
What's your most trivial piece of knowledge?
ps. I've been sorting through photos and am almost ready to start posting catch up posts about my recent travels! If anyone has good ways to organize and work with photos on an iphone, please tell me!