There is a debate within Orthodox Judaism as to whether the Oral Tora, (Toshba) is an explanation of the Written Torah, helping us understand the Tora's original meaning, or whether the Oral Tora is more than an explanation, and is actually an independent entity, bound to the Written Tora but with equal and independent authority.
Proponents of the first view -- that the Oral Tora is an explanation of the Written Tora -- could, for example, cite the Mitzvah of Tefillin in defense of their claim: We are told to bind "these words between our eyes and on our hands", but we only know from the Oral Tora that this means Tefillin on our foreheads and arms. Thus the Oral Tora comes to explain the words of a verse that would otherwise be unintelligible to us.
Proponents of the second view -- that the Oral Tora is a separate entity, bound to the Written Torah but with equal and independent authority -- might cite the 39 Melachot of Shabat as an example of their claim: Traditionally, the prohibitions of the Shabat are linked to the work that was done on the Mishkan. But nowhere in Sefer Shemot are 39 categories of labor clearly mentioned when building the Mishkan. The only clear textual link between the Mishkan and Shabat is the juxtaposition of the narrative(s) regarding the building of the Mishkan with the Commandment(s) to observe Shabat. The idea of 39 Melachot of Shabat is first introduced in the Gemarah. The Torah does explicitly forbid work, lighting a fire (lo tevaaru esh), and collecting wood and manna (I think -- forming the basis for eruvin and prohibitions of cooking). Chazal, however, taught and passed on most of the melachot of the mishkan, and their corresponding prohibitions on Shabat, without having clear textual references for them in the Chumash.
So the question -- according the second view -- is: how strong is the link between the Oral Tora and the Written Tora? How independent is the Gemarah of the Chumash? Isn't it dangerous to give equal authority to the Oral Tora, which is continuous, dynamic and evolving today, as to the Written Tora, which is a sealed Book?
Yishayahu Leibovitz, who discusses this issue in his commentary of the Chumash, quotes the Baal Haturim on Parshat Vayakhel. Here, in a subtle but tremendously significant observation, he points out that the 40th word (39 + 1) of Parshat Vayakhel, (from Shemot 35:1) is -- Shabat! (Check it out for yourself.)
Of course, some will put this down to mere chance. An irrelevant fluke. And who could disprove them? But others -- dare I say believers -- can see much, much more here. What they can see is that the idea of 39 Melachot, that formative characteristic of what Shabat is in the Talmud, is hinted at, albeit mysteriously, in the Chumash! It is a remarkable example of how truly intertwined the Oral and Written Toras are.