In regards to borer on shabbos, lets say one had two foods, one of which he likes, the other he does not. If he accidently separated out the food that he doesnt like, then realizes the error, can he eat this food and solve the problem of borer? Will the mistake be retroactively erased in this way?
It appears that eating the food that was separated will not help.
At the time it was separated it was taken as pesoles, and not as ochel. Just as mixing the foods again won’t help to fix the prohibition, so eating the food won’t fix the prohibition.
This idea emerges from the ruling of the Chafetz Chayim (Sha’ar Ha-Zion 319:5), who writes that after a person separates in a permitted way, for the purpose of eating right away, there is no longer any obligation to actually eat, and even if one leaves the food no prohibition is transgressed.
However, the Peri Megadim (M.Z. 319:2) writes the contrary, stating that the food that was separated (in a permitted way) must be eaten, and if it isn’t eaten a prohibition is retroactively transgressed.
Therefore, according to the Peri Megadim it appears that one can save the day by eating the separated food.
Although this approach appears difficult, and the Mishnah Berurah does not mention it, it is therefore recommended to eat the separated food, for according to the Peri Megadim this will mend the transgression.