There are fascinating discrepancies between the two accounts of preparing the korban Pesach in the Torah.
Exodus 12:3, 12:9:
דַּבְּרוּ, אֶל-כָּל-עֲדַת יִשְׂרָאֵל לֵאמֹר, בֶּעָשֹׂר, לַחֹדֶשׁ הַזֶּה: וְיִקְחוּ לָהֶם, אִישׁ שֶׂה לְבֵית-אָבֹת--שֶׂה לַבָּיִת.
Speak unto all the congregation of Israel, saying: In the tenth day of this month they shall take to them every man a lamb, according to their fathers' houses, a lamb for a household;
אַל-תֹּאכְלוּ מִמֶּנּוּ נָא, וּבָשֵׁל מְבֻשָּׁל בַּמָּיִם: כִּי אִם-צְלִי-אֵשׁ, רֹאשׁוֹ עַל-כְּרָעָיו וְעַל-קִרְבּוֹ.
Do not eat it raw, nor boiled with water, but roast it with fire; its head with its legs and with the inwards thereof.
Much later (according to tradition almost 40 years later!), in Deuteronomy 16:2, 16:7, it goes like this:
וְזָבַחְתָּ פֶּסַח לַיהוָה אֱלֹהֶיךָ, צֹאן וּבָקָר, בַּמָּקוֹם אֲשֶׁר-יִבְחַר יְהוָה, לְשַׁכֵּן שְׁמוֹ שָׁם.
And you shall sacrifice the passover offering unto Hashem your God, of the flock and the herd, in the place which Hashem shall choose to cause His name to dwell there.
וּבִשַּׁלְתָּ, וְאָכַלְתָּ, בַּמָּקוֹם, אֲשֶׁר יִבְחַר יְהוָה אֱלֹהֶיךָ בּוֹ; וּפָנִיתָ בַבֹּקֶר, וְהָלַכְתָּ לְאֹהָלֶיךָ.
And you shall roast and eat it in the place which the Hashem your God shall choose; and you shall turn in the morning, and go unto your tents.
Notice the differences? In the Exodus account, one is instructed to take a lamb and roast it in fire, making sure not to get cook it with water. In the Deuteronomy account, one is instructed to take from the flock or the herd, and to boil it! Mechon Mamre incorrectly renders וּבִשַּׁלְתָּ as "And you shall roast" instead of "And you shall cook (or boil)". (Regardless, kol hakavod to Mechon Mamre as well as to the inventor of cut & paste - it would take me forever to type the Hebrew!)
So how do we know that the Exodus account is the method by which one is to prepare the Korban Pesach? Rabbi Ishmael b. Elisha's 13th rule of exegesis: "When two Biblical passages contradict each other, the contradiction is be solved by reference to a third passage." And the third passage is Exodus 12:21:
וַיִּקְרָא מֹשֶׁה לְכָל-זִקְנֵי יִשְׂרָאֵל, וַיֹּאמֶר אֲלֵהֶם: מִשְׁכוּ, וּקְחוּ לָכֶם צֹאן לְמִשְׁפְּחֹתֵיכֶם--וְשַׁחֲטוּ הַפָּסַח.
Then Moses called for all the elders of Israel, and said unto them: 'Draw out, and take you lambs according to your families, and kill the passover lamb'.
Problem resolved - we use a lamb and not an animal from the herd.
But wait - what about boiling vs. roasting?? What pasuk do we use to resolve this contradiction?
There is none, and that's a problem, for according to normative halacha one is prohibited from deriving any benefit from a cooked korben Pesach (not just eating)! Rashi comments on וּבִשַּׁלְתָּ and states "This is roasting by fire, for it too is called cooking". Ibn Ezra also sees fit to simply mention that the pasuk is talking about roasting.
The Sapirstein Edition of Art Scroll has a footnote to the Rashi, pointing out a Mechilta that references 2 Chronicles 35:13:
וַיְבַשְּׁלוּ הַפֶּסַח בָּאֵשׁ, כַּמִּשְׁפָּט; וְהַקֳּדָשִׁים בִּשְּׁלוּ, בַּסִּירוֹת וּבַדְּוָדִים וּבַצֵּלָחוֹת, וַיָּרִיצוּ, לְכָל-בְּנֵי הָעָם
And they boiled the Pesach with fire according to the ordinance; and the holy offerings they boiled in pots, in kettles, and in pans, and carried them quickly to all the children of the people.
implying that "bishul" can also mean roasting. But attempting to reconcile a Torah contradiction by redefining the common use of a word seems to me to be a very weak answer. At most, one can only say that Chronicles states a tradition that was in force at the time of its writing.
A proponent of the Documentary Hypothesis has no problem with these contradictions - the two passages simply different traditions were only harmonized much later. Mark Zvi Brettler in How to Read the Jewish Bible discusses this briefly, stating that the account in Chronicles
may be an attempt to cover up ... two irreconcilable traditions. Centuries later, the process just delineated - where two different Torah texts from two different sources are 'reconciled' - becomes a key process in rabbinic midrash. The rabbis did not recognize that the Torah is comprised of sources - to them, it is a single holy text, given by God to Moses. Thus, they too need to reconcile what we see as source-critical differences. It starts as soom as editors combine the Torah's sources and a community canonizes the that text. That process has already taken place by the time of the Chronicler.
David Weiss Halivni in Revelation Restored: Divine Writ and Critical Responses asks how the decision between conflicting prescriptions was made and gives two possibles answers: either "details and decisions were a matter of personal choice" and the "resolutions indicated by the Chronicles would represent, not a priori decisions, but accounts, after the fact, what the people had chosen to do" OR "that some authoritative determinant, beyond the written text, was responsible for consistent and coherent behavior". Halivni says that only the second alternative is plausible, since multiple traditions would not have been appealing to the people at the time of Ezra who were zealous to hear authoritative guidance to the Law. But neither does Halivni posit that the reconciliation was an oral tradition dating back to Moses, he recognizes that the Torah text is maculate, and states that the same people (Ezra et al) who presented the scriptural canon to Israel were also responsible for interpreting how the contradictions were to be resolved!
As an aside, note also that the Exodus and Deuteronomy passages - along with Leviticus 23:6 - are used in the well-known 6-days vs 7-days discrepancy to prove that the obligation to eat matzah is only on the first day of Pesach and the other six days are a rishus. Readers may draw their own conclusion from the contradictions and the resolution.