Comment #2 on issue 3761 by snoob...@yahoo.ie: overlappingPairsCollect: fails on OrderedCollections http://code.google.com/p/pharo/issues/detail?id=3761 Need to change #at:put: to rather be #add:. The reason for this is explained in comment of OrderedCollection>>at:put: "Put anObject at element index anInteger. at:put: cannot be used to append, front or back, to an ordered collection; it is used by a knowledgeable client to replace an element." Change could be as given below... overlappingPairsCollect: aBlock "Answer the result of evaluating aBlock with all of the overlapping pairs of my elements." | retval | retval := self species new: self size - 1. 1 to: self size - 1 do: [:i | retval add: (aBlock value: (self at: i) value: (self at: i + 1)) ]. ^retval