Don't expect class destructors to be called at all by the GC

[Mike Franklin via Digitalmars-d-learn]

"Don't expect class destructors to be called at all by the GC"
I was a bit shocked to read that here: 
https://p0nce.github.io/d-idioms/#The-trouble-with-class-destructors

The document tries to clarify with:
"The garbage collector is not guaranteed to run the destructors 
for all unreferenced objects."

Unfortunately, that doesn't really shed much light on this 
oddity.  So, specifically, under what circumstances are 
destructors not called?

Thanks,
Mike