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Chametz that is found on a high surface such as on top of a closet or bookcase:
One is obligated to destroy a Kezayis of Chametz even if it is found on a very high surface. [Furthermore even less than a Kezayis should be destroyed.] The above obligation applies even if one already nullified his Chametz. Thus, one is obligated to bring a ladder on the night of the 14th [or prior to it] and take the Chametz down from on top of his closets, dressers, book cases and the like.
 Admur 438:11
 The reason: The reason for this is because it is possible for the Chametz to fall and one may then forget and come to eat it. [Admur ibid] It is thus not similar to Chametz found in cracks and crevices that are unreachable by hand, as there is no worry that such Chametz will become reachable over Pesach.
 Although in 438:11 Admur writes “a kezayis” is to be destroyed from a high surface [and not less-as rules the Elya Raba 438:7 and Olas Shabbos 438:2] nevertheless in the Kuntrus Acharon he questions this ruling and seemingly leans to be stringent.