Last week I devoted a post or two to the issue of eating a ta'aroves chameitz. There is more to chameitz, however, than eating. On the first mishna in "Eilu Ovrin", Rashi writes that ta'aroves chameitz is also subject to the issurim of bal yera'eh u'bal yimatzei. Tosfos disagrees, arguing that "kol machmetzes" which prohibits ta'aroves is a specific issur achila, but we have no makor that would indicate ta'aroves chameitz is subject to bal yera'eh u'bal yimatzei.
The Kesef Mishna questions according the Rashi's view, shared by the Rambam, how much chameitz is in this ta'aroves? Some explain that the Rambam is referring only to a k'zayis b'kedei achilas pras of chameitz in a mixture. Any mixture which contains a kzayis b'kdei achilas pras of issur is the same as the issur itself (i.e. not in a mixture). [Note: see last week's post on the Rambam's unique shita by ta'aroves chameitz]. However, the KS"M suggests that even if there is less than a k'zayis b'kdei achilas pras there would still be an issur of bal yera'eh. Even though if you eat such a ta'aroves you cannot consume a kzayis in the time frame that would result in an issur achila, with respect to bal yera'eh we look at the sum total of the mixture - does this mixture as a whole contain a k'zayis in it, irrespective of how long it would take to consume it?
What emerges from the KS"M is an interesting chiddush - with respect to achila, there can be so little chamietz that is it bateil in the mixture, but with respect to bal yera'eh the same chameitz in the same mixture creates an issur . We do find similar issues elsewhere - in Bechoros 23 the gemara discusses neveila being bateil to shechuta with respect to removing an issur achila from the mixture, but tumas neveila still applies (see Tos.) The Pri Megadim (intro to 452) discusses this concept at length (meaning, too much to blog about : )