Discussing this with Radoslav Hodničák on the Parcplace.net #smalltalk
channel turned up that this could be due to blocks not being real
closures in Squeak. Sending #fixTemps to the block that has [:x |...]
seems to prevent the crash.

"<rh> process 1 is invoked with x=1, runs into critical, adds 1 to
collection, get preempted. process 2 is invokes with x=2 (x is now 2
for all processes), runs into critical block, stops, processes 1 now
tries to remove 2 and fails"

which makes sense.