The Gemara [Eruvin 21b] says that if one transgresses the words of the rabbis, he is חייב מיתה.
The Maharsha explains that this only refers to an instance where the rabbis made a fence for a biblical law for which one is חייב מיתה. Otherwise, it wouldn't make sense that for the actual law one is not חייב מיתה but for not keeping the fence meant to guard the mitzva, one is. The טפל [the דרבנן] can't be more severe than the עיקר [the דאורייתא].
This thesis seems to be debunked by an open Gemara [Brachos 4b]:
לְעוֹלָם כְּרַבַּן גַּמְלִיאֵל סְבִירָא לְהוּ, וְהָא דְּקָא אָמְרִי עַד חֲצוֹת, כְּדֵי לְהַרְחִיק אֶת הָאָדָם מִן הָעֲבֵירָה. כִּדְתַנְיָא: חֲכָמִים עָשׂוּ סְיָיג לְדִבְרֵיהֶם, כְּדֵי שֶׁלֹּא יְהֶא אָדָם בָּא מִן הַשָּׂדֶה בָּעֶרֶב וְאוֹמֵר: אֵלֵךְ לְבֵיתִי וְאוֹכַל קִימְעָא וְאֶשְׁתֶּה קִימְעָא וְאִישַׁן קִימְעָא, וְאַחַר כָּךְ אֶקְרָא קְרִיאַת שְׁמַע, וְאֶתְפַּלֵּל, וְחוֹטַפְתּוֹ שֵׁינָה, וְנִמְצָא יָשֵׁן כָּל הַלַּיְלָה. אֲבָל אָדָם בָּא מִן הַשָּׂדֶה בָּעֶרֶב, נִכְנָס לְבֵית הַכְּנֶסֶת. אִם רָגִיל לִקְרוֹת, קוֹרֵא, וְאִם רָגִיל לִשְׁנוֹת, שׁוֹנֶה, וְקוֹרֵא קְרִיאַת שְׁמַע וּמִתְפַּלֵּל, וְאוֹכֵל פִּתּוֹ וּמְבָרֵךְ. וכל העובר על דברי חכמים - חייב מיתה.
The Gemara answers: Actually, the Rabbis hold in accordance with the opinion of Rabban Gamliel, and the fact that they say until midnight is in order to distance a person from transgression. As it was taught in a braisa, the Rabbis created a “fence” for their pronouncements with regard to the recitation of Shema in order to prevent a situation where a person comes home from the field in the evening, tired from his day’s work, and knowing that he is permitted to recite Shema until dawn says to himself: I will go home, eat a little, drink a little, sleep a little and then I will recite Shema and recite the evening prayer. In the meantime, he is overcome by sleep and ends up sleeping all night. However, since one is concerned lest he fall asleep and fail to wake up before midnight in order to recite Shema at the appropriate time, he will come from the field in the evening, enter the synagogue, and until it is time to pray, he will immerse himself in Torah. If he is accustomed to reading the Bible, he reads. If he is accustomed to learning mishnayos, a more advanced level of study, he learns. And then he recites Shema and prays as he should. When he arrives home, he eats his meal and makes the bracha. And anyone who transgresses the words of the Sages, is liable to the death penalty.
So we see that even when we are not dealing with a law that mandates the death penalty, one who is עובר על דברי חכמים is חייב מיתה???!!!!
Maybe: In this case even the Maharsha would agree that he is חייב מיתה b/c according to תלמידי רבינו יונה if one doesn't read Shema by Chatzos, he shouldn't read at all. So here the rabbis UPROOTED the mitzva if you don't perform it as they mandated. So here the טפל IS more חמור than the עיקר. The עיקר מצוה according to the Torah may be performed all night. The rabbis overrode this and said only until Chatzos. So just like the רבנן is so חמור in that sense, so is the punishment more חמור.
[מרן ראש הישיבה זצ"ל]
