The following test program will, in roughly 50% of its runs on my machine, generate the following backtrace immediately after the first "Global garbage collection... done" message. In the other runs, the global collection message prints and the program continues running without the backtrace. I'm not sure if the backtrace has caused any of the three Processes to exit.

Backtrace:

~$ gst threading-preemption.st
"Global garbage collection... done"
Object: nil error: did not understand #goodness:
MessageNotUnderstood(Exception)>>signal
UndefinedObject(Object)>>doesNotUnderstand: #goodness:
optimized [] in BlockClosure class>>exceptionHandlerSearchBlock
[] in Kernel.CoreException>>instantiateNextHandler:
MethodContext(ContextPart)>>scanBacktraceForAttribute:do:
Kernel.CoreException>>instantiateNextHandler:
MessageNotUnderstood(Exception)>>signal
UndefinedObject(Object)>>doesNotUnderstand: #key
Delay class>>unscheduleDelay:
Delay class>>handleDelayEvent
optimized [] in Delay class>>runDelayProcess
[] in BlockClosure>>ifCurtailed:
BlockClosure>>ensure:
BlockClosure>>ifCurtailed:
Delay class>>runDelayProcess
optimized [] in Delay class>>startDelayLoop
[] in Process>>onBlock:at:suspend:
BlockClosure>>on:do:
[] in Process>>onBlock:at:suspend:
BlockClosure>>ensure:
[] in Process>>onBlock:at:suspend:
[] in BlockClosure>>asContext:
BlockContext class>>fromClosure:parent:

Code:

"Attempt to use a high-priority process to preempt lower-priority
processes. Doesn't work."

[
  | d |
  d := Delay forMilliseconds: 10.
  [
    d wait.
  ] repeat.
] forkAt: Processor highestPriority.

[
  [
    Transcript show: 'one'.
    (Delay forMilliseconds: 10) wait.
  ] repeat.
] fork.

[
  [
    Transcript show: 'two'.
    (Delay forMilliseconds: 10) wait.
  ] repeat.
] value.