a := OrderedCollection with: 1 with: 2.
b := OrderedCollection with: 3 with: 4.
a with: b collect: [:x :y | x + y]

produces a walkback, because with:collect: creates a new, empty OrderedCollection
to hold the answer, then tries to at:put: the answers there. But an empty ordered
collection has no place at which to put them. :-)

-- 
Stan