Sorry, the comments I wrote got lost somehow:

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The message #atRandom is not thread-save when being used concurrently on 
different collections or integers. As a test try to evaluate the 
following expressions in your image, the random generator provided by 
Collection gets corrupted:

	[ [ (1 to: 100) atRandom ] repeat ] forkAt: 30.
	[ [ 100 atRandom ] repeat ] forkAt: 30.

This is a critical bug! As an example the superb web-framework Seaside 
uses random key-generation to identify its sessions. If other threads in 
your image use #atRandom problems might arise.

The attached change-set solves the problem by introducing a critical 
section to the Collection class providing the random generator.

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Cheers,
Lukas & Adrian

-- 
Lukas Renggli
http://renggli.freezope.org