Vessels used throughout the year with hot ĥametz foods cannot be used during Pesaĥ since heat causes vessels to absorb the taste of ĥametz. In order to use such utensils during Pesaĥ, one must first remove the taste of the ĥametz through hagala or libun.
If one cooks in such a pot on Pesaĥ knowing that it had not been koshered, even if twenty-four hours have passed and the taste of the ĥametz has been befouled, the cooked food is forbidden. As we have learned, the Sages forbade such food in order to penalize people who deliberately cook in vessels that have absorbed ĥametz and have not been koshered.
What if one errs and cooks in a pot that was not koshered for Pesaĥ? According to Shulĥan Arukh, if twenty-four hours have elapsed since the ĥametz was cooked in the pot, the food may be eaten on Pesaĥ, because the taste of the released ĥametz is foul. According to Rema, the food is forbidden even though the ĥametz is foul because the prohibition of ĥametz is especially severe, for even a drop of ĥametz causes a food to be forbidden (SA 447:10; see above 7:5).
Ĥametz utensils that one does not wish to make kosher for Pesaĥ must be cleansed of any residual ĥametz and put away in a closed place so that nobody inadvertently uses them during Pesaĥ (SA 451:1; see also above 6:4, where we learned that such utensils should not be sold).