When you run TerminationExperiment's #run, it can sometimes reach several tens of thousands of iterations of busyLoop before faulting, but will often fault before 20,000 iterations have completed on my machine.

When you run #run2, things behave as I would have expected them to behave. No faulting occurs, and the process is sensibly terminated.

Here's a selection of some of the faults I see from #run after several seconds. Each comes at the end of a single run of the script, as attached.

Object: '' error: Invalid argument : must be a SmallInteger
SystemExceptions.WrongClass(Exception)>>signal
SystemExceptions.WrongClass class>>signalOn:mustBe:
SystemExceptions.WrongClass class>>signalOn:mustBe:
String(Object)>>checkIndexableBounds:put:
String>>at:put:
String class(ArrayedCollection class)>>with:
Character>>asString
[] in TextCollector>>showCr:
[] in Semaphore>>critical:
BlockClosure>>ensure:
Semaphore>>critical:
TextCollector>>showCr:
String(Object)>>printNl
TerminationExperiment>>busyLoop
TerminationExperiment>>run
UndefinedObject>>executeStatements

Object: nil error: did not understand #generality
MessageNotUnderstood(Exception)>>signal
UndefinedObject(Object)>>doesNotUnderstand: #generality
SmallInteger>>+
FileStream(FileDescriptor)>>write:from:to:
FileStream>>flush
FileStream(Stream)>>nextPutAllFlush:
TextCollector>>primNextPutAll:
[] in TextCollector>>showCr:
[] in Semaphore>>critical:
BlockClosure>>ensure:
Semaphore>>critical:
TextCollector>>showCr:
String(Object)>>printNl
TerminationExperiment>>busyLoop
TerminationExperiment>>run
UndefinedObject>>executeStatements

Object: String error: Invalid argument 'b': must be a SmallInteger
SystemExceptions.WrongClass(Exception)>>signal
SystemExceptions.WrongClass class>>signalOn:mustBe:
SystemExceptions.WrongClass class>>signalOn:mustBe:
String class(Behavior)>>new:
String(Collection)>>copyEmpty:
String(ArrayedCollection)>>copyFrom:to:
WriteStream>>contents
String(Object)>>printString
String(Object)>>printNl
TerminationExperiment>>busyLoop
TerminationExperiment>>run
UndefinedObject>>executeStatements

Object: 3 error: did not understand #replaceFrom:to:with:startingAt:
MessageNotUnderstood(Exception)>>signal
SmallInteger(Object)>>doesNotUnderstand: #replaceFrom:to:with:startingAt:
String(ArrayedCollection)>>copyFrom:to:
WriteStream>>contents
String(Object)>>printString
String(Object)>>printNl
TerminationExperiment>>busyLoop
TerminationExperiment>>run
UndefinedObject>>executeStatements

This one includes the end of the stdout, to illustrate an odd doubling
of 'c':

'a'
'b'
'c''c'Object: nil error: did not understand #generality
MessageNotUnderstood(Exception)>>signal
UndefinedObject(Object)>>doesNotUnderstand: #generality
SmallInteger>>+
FileStream(FileDescriptor)>>write:from:to:
FileStream>>flush
FileStream(Stream)>>nextPutAllFlush:
TextCollector>>primNextPutAll:
[] in TextCollector>>showCr:
[] in Semaphore>>critical:
BlockClosure>>ensure:
Semaphore>>critical:
TextCollector>>showCr:
String(Object)>>printNl
TerminationExperiment>>busyLoop
TerminationExperiment>>run
UndefinedObject>>executeStatements

Object: String error: did not understand #copyEmpty:
MessageNotUnderstood(Exception)>>signal
String class(Object)>>doesNotUnderstand: #copyEmpty:
WriteStream>>growCollection
WriteStream>>nextPut:
String>>printOn:
String(Object)>>printString
String(Object)>>printNl
TerminationExperiment>>busyLoop
TerminationExperiment>>run
UndefinedObject>>executeStatements

etc. etc.