A senryū from Yamazawa Hideo 山澤英雄's Yanagidaru meiku sen (ed. Kasuya Hiroki 粕谷宏紀 1995, Iwanami Shoten):

虫よけをよみよく張るは無筆なり
mushiyoke wo/ yomi-yoku haru wa/ muhitsu nari
Those who paste up insect-repelling poems in a way that makes them easy to read, can't read.

Kasuya explains in a footnote that Edoites believed that insects could be kept out of the bathroom, kitchen, etc. by pasting up a copy of the poem "千早ふる卯月八日は吉日よかみさけ虫の成敗ぞする" ("The marvelous eighth of the fourth lunar month is an auspicious day; flies will be punished severely") written in ink mixed with the sweet tea served by temples on Buddha's birthday (the eighth of the fourth, you see). For real efficacy, you apparently wanted to paste the poem upside-down, or at least have the 虫 ("insect") character inverted.

As a result, hanging your insect-repelling poem the right way up reveals that you do not actually know which side is up, in the sense that the characters are just squiggles t you.

Kasuya further notes that this practice can be observed in the illustrations in Santō Kyōden 山東京伝's Edo umare uwaki no kabayaki (江戸生艶気樺焼, "Playboy, Roasted à la Edo"*) — and it's true! In fact, thanks to the magic of Waseda University's online Edo literature collection, we can confirm this without even standing up. The book is here; the poem can be seen at the bottom right of page 9, above a pair of geta.

Note also that the word corresponding to "illiterate" isn this senryū is 無筆, literally (!) "brushless". Etymologically speaking, the conceptual divide is the ability to produce letters, not just understand them.