Here is a copy of something I wrote about this earlier:

In Pharo 1.3, SequenceableCollection>>combinations:atATimeDo: will,
for all combinations, send the same collection instance as argument
for the block. Therefore, if you use this collection itself, you will
be surprised:

|answer|
answer := OrderedCollection new.
#(a b c) combinations: 2 atATimeDo: [:each | answer add: each].
answer

Here I expect to end up with a collection looking like this:
1: �#(#a #b)
2: �#(#a #c)
3: �#(#b #c)

But I end up with:
1: �#(#c #c)
2: �#(#c #c)
3: �#(#c #c)

One fix is to change the client code to copy the argument for the block:
|answer|
answer := OrderedCollection new.
#(a b c) combinations: 2 atATimeDo: [:each | answer add: each copy].
answer

Another solution is enhancing
SequenceableCollection>>combinationsAt:in:after:do: to copy the
collection:
nn + 1 to: self size do: [ :index |

� � � � � � � aCollection at: jj put: (self at: index).
� � � � � � � jj = aCollection size

� � � � � � � � � � � ifTrue: [ aBlock value: aCollection copy ]

� � � � � � � � � � � ifFalse: [
� � � � � � � � � � � � � � � self
� � � � � � � � � � � � � � � � � � � combinationsAt: jj + 1
� � � � � � � � � � � � � � � � � � � in: aCollection
� � � � � � � � � � � � � � � � � � � after: index
� � � � � � � � � � � � � � � � � � � do: aBlock ] ]

Kind regards
Runar Jordahl