From a defense of the Old Believer practice of prostrating on Sunday, a useful reminder:
Sometimes, the individual doesn’t know any canons, but are repeating something they heard and assume to be true. Other times, they may know a few of the major ones that they know other groups violate - such as the canons on the dating of Pascha - that it cannot precede or fall on the Jewish Passover, which they know from the problems of the Gregorian Calendar. This is another odd one to know, but also demonstrates that the known canons are not the ones by which the questioner orders his own life, but those by which he can find violations in other faiths. Pharisaism indeed.
(Emphasis mine.)