Prof. Mark Liberman (Language Log) points to the little-known history of the word “syllabus.” Apparently the Greek original is “sittybas,” from “sittyba,” “parchment label or title-slip on a book” — but then an early printing of Cicero mistranscribed the word as “syllabos.” (Go to Liberman’s post for more, including a reproduction of the OED entry.) That’s the way language works: Errors, accepted long enough, become standard.