1) "DAVAR SHE'YESH LO MATIRIN"
OPINIONS: The Gemara states that an egg laid on Yom Tov is a "Davar she'Yesh Lo Matirin," an item that eventually will become permitted, and therefore if it becomes mixed with any number of normal, permitted eggs, the entire mixture is prohibited. The Gemara concludes that a "Davar she'Yesh Lo Matirin" does not become annulled in a mixture even if it is only an Isur d'Rabanan that became mixed with permitted items (such as in the case of an egg laid on Yom Tov). Furthermore, even if the item is a Safek Isur d'Rabanan (such as when it is not known whether the egg was laid today, on Yom Tov, or whether it was laid on a weekday, before Yom Tov), it is not permitted if it is a "Davar she'Yesh Lo Matirin." The normal principle of "Safek d'Rabanan l'Kula" does not apply to permit the item.
What is the logic behind the Rabanan's enactment to prohibit a "Davar she'Yesh Lo Matirin"?
(a) RASHI (3b, DH Afilu) explains that since one will be able to eat the item when it becomes permitted (after Yom Tov), he should not rely on Bitul or on the principle of "Safek d'Rabanan l'Kula" in order to eat it on Yom Tov.
Rashi's reasoning may be related to the Machlokes Rishonim in the case of one piece of Isur which fell into two pieces of Heter and then the mixture was cooked. In such a case, does the Isur prohibit the entire mixture because the cooking process causes all of the permitted parts to absorb some of the taste of the forbidden item, or does the mixture remain permitted because the Isur became annulled when it initially fell into a majority of Heter? The RASHBA (Chulin 100a) rules that the mixture is prohibited. The ROSH (Chulin 7:37) rules that after the Isur was annulled at the moment it fell into the Heter, there remains no element of Isur in the mixture and the mixture cannot become prohibited when cooked.
Rashi may follow the view of the Rashba. Rashi says that even though the forbidden item is mixed with enough items of Heter to become annulled, since the item is a "Davar she'Yesh Lo Matirin" one should not rely on Bitul and he should wait until it becomes permitted. Rashi apparently maintains that even when an item of Isur fell into enough items of Heter to be Mevatel it, there still remains an element of Isur in the mixture.
(b) The RAN in Nedarim (52a) explains that the reason why, normally, an item of Isur becomes annulled in a mixture with items of Heter is because when opposites combine they contrast against each other and annul (whichever one is the majority is Mevatel the one which is the minority). This is the mechanism behind the concept of Bitul. When like items combine they cannot be Mevatel each other because there is no contrast.
Normally, when an item of Isur becomes mixed with Heter, one annuls the other. Even though the two items are the same type of food, they contrast because one is Asur and one is Mutar, and thus they are considered opposites. However, if an item is Asur now and will become Mutar later, it cannot become annulled when it falls into Heter because there is not enough opposition; it is as if the item of Isur (which will become Mutar later) is Mutar right now. This is why the stringency of a "Davar she'Yesh Lo Matirin" exists only in a case of a mixture of Min b'Mino -- two of the same types of food (such as an egg that is Asur which became mixed with many eggs that are Mutar). In a case of a mixture of Min b'she'Eino Mino (two different types of foods), even if the Isur will become Mutar later there is still enough opposition to be Mevatel it due to the different nature of the foods themselves. In a mixture of Min b'Mino, the only opposition is the fact that one item is Asur and the others are Mutar. (See also TESHUVOS HA'RAN #51.)
According to Rashi's reasoning, why does the stringency of "Davar she'Yesh Lo Matirin" not apply to a mixture of Min b'she'Eino Mino? Perhaps it is because in a case of Min b'Mino, the taste remains the same, and thus one who eats the mixture actually tastes the flavor of the prohibited item. In a case of Min b'she'Eino Mino the taste is that of the permitted food, and therefore the taste of the Isur is Batel; there is less reason to prohibit one from eating the mixture now even though he can eat it at a later time without relying on Bitul. (See SHACH YD 102:5, who cites the REMA's answer to this question. The answer suggested here for Rashi may be the intention of the Rema.)
The Acharonim point out that the Ran's explanation does not suffice to explain why the stringency of "Davar she'Yesh Lo Matirin" applies to a Safek d'Rabanan (see TESHUVOS REBBI AKIVA EIGER 1:189). A Safek d'Rabanan is an item which might be Asur mid'Rabanan. It does not involve any mixture. Since there is no question of Bitul (which applies, of course, only in a case of a mixture), there is no opposition-item to annul the Isur. It must be that the Ran agrees with the logic expressed by Rashi. Why, then, does he add his own explanation for the logic of "Davar she'Yesh Lo Matirin"? Perhaps the Ran maintains that Rashi's logic suffices only to explain why the stringency of "Davar she'Yesh Lo Matirin" applies in the case of a Safek d'Rabanan. It does not explain why the stringency applies in the case of a mixture. There is no point in waiting for the Isur to become Mutar, because the item of Isur has already become annulled and is no longer considered an item of Isur (as mentioned above).