I found automated way how to check problem:

p := [ Object basicSize ] fork.

p suspend.

3 timesRepeat: [p step: p suspendedContext].

newContext := p completeStep: p suspendedContext.

newContext selector. "=> doesNotUnderstand:"

newContext arguments first selector == 0

2016-12-05 15:08 GMT+01:00 Denis Kudriashov <dionisiydk@gmail.com>:

2016-12-05 15:07 GMT+01:00 Denis Kudriashov <dionisiydk@gmail.com>:
I attach crash file.
When I try it second time. After step over I got DNU error which shows very strange thing that 0 integer was sent as message. And when I click on basicSize method image crashed.
Then I try to get same behaviour but can't. Image crash when I press step over debugger button. (dump file about this case)

It is on one of latest pharo 6 image and mac os VM (dump includes this information I guess)