(I originally asked this in Christianity.SE, but was redirected since the question deals with Jewish laws. Please forgive my obvious ignorances of Jewish society.)
In the Christian Bible, Matthew 15:2, the Pharisees accused the disciples of Jesus of not properly washing their hands before eating.
“Why do your disciples break the tradition of the elders? For they do not wash their hands when they eat.”
What, if any, penalty or punishment could they have received for this?
The Talmud in Berachot 19a mentions that someone was excommunicated for making light of the handwashing ritual:
R. Joshua b. Levi further said: In twenty-four places we find that the Beth din inflicted excommunication for an insult to a teacher, and they are all recorded in the Mishnah. R. Eleazar asked him, Where? He replied: See if you can find them. He went and examined and found three cases: one of a scholar who threw contempt on the washing of the hands, another of one who made derogatory remarks about scholars after their death, and a third of one who made himself too familiar towards heaven. What is the case of making derogatory remarks about scholars after their death? — As we have learnt: He used to say: The water [of the sotah] is not administered either to a proselyte or to an emancipated woman; the Sages, however say that it is. They said to him: There is the case of Karkemith an emancipated bondwoman in Jerusalem to whom Shemaiah and Abtalyon administered the water? He replied: They administered it to one like themselves. They thereupon excommunicated him, and he died in excommunication, and the Beth din stoned his coffin. What is the case of treating with contempt the washing of the hands? — As we have learnt: R. Judah said: Far be it from us to think that Akabiah b. Mahalalel was excommunicated, for the doors of the Temple hall did not close on any man in Israel the equal of Akabiah b. Mahalalel in wisdom, in purity and in fear of sin. Whom did they in fact excommunicate? It was Eleazar b. Hanoch, who raised doubts about washing the hands, and when he died the Beth din sent and had a large stone placed on his coffin, to teach you that if a man is excommunicated and dies in his excommunication, the Beth din stone his coffin.
(Soncino translation, my emphasis)
There's also a story in Yoma 83b about the negative consequences that could come from failure to wash hands:
Also, R. Meir and R. Judah and R. Jose were on a journey together. (R. Meir always paid close attention to people's names, whereas R. Judah and R. Jose paid no such attention to them). Once as they came to a certain place. they looked for a lodging, and as they were given it, they said to him [the innkeeper]: What is your name? — He replied: Kidor. Then he [R. Meir] said: Therefrom it is evident that he is a wicked man, for it is said: For a generation [ki-dor] very forward are they. R. Judah and R. Jose entrusted their purses to him; R. Meir did not entrust his purse to him, but went and placed it on the grave of that man's father. Thereupon the man had a vision in his dream [saying]: Go, take the purse lying at the head of this man! In the morning he [the innkeeper] told them [the Rabbis] about it, saying: This is what appeared to me in my dream. They replied to him: There is no substance in the dream of the Sabbath night . R. Meir went, waited there all day, and then took the purse with him. In the morning they [the Rabbis] said to him,: ‘Give us our purses’. He said: There never was such a thing! R. Meir then said to them: Why don't you pay attention to people's names? They said: Why have you not told this [before]. Sir? He answered: consider this but a suspicion.I would not consider that a definite presumption! Thereupon they took him [the host] into a shop [and gave him wine to drink]. Then they saw lentils on his moustache. They went to his wife and gave her that as a sign, and thus obtained their purses and took them back. Whereupon he went and killed his wife. It is with regard to this that it was taught: [Failure to observe the custom of] the first water caused one to eat the meat of pig, [failure to use] the second water slew a person.
(Soncino translation, my emphasis)
There is no penalty from human justice. The Talmud teaches that there is a penalty from heaven.
However, the automatic punishment is frequently death in times of plagues and epidemics. During the Black Plague in 14th-century Europe, the incidence was much lower among the Jews, because they washed their hands before eating bread and after going to the bathroom, bathed before Shabbat, washed corpses before burial, etc. This, of c(o)urse, made antisemitism spike.
Something to ponder these days.
