Thanks for this article, I found it quite useful.
I ran into this when writing a test for a package I wrote to ensure that it properly bombed with tainted data. The package wrapped an Unix command, and forked.
I was quite confused when Test::Harness started showing duplicate results for the same test number (one passing and one failing:). Quite a bit of head banging until I could reduce the issue to something which lead me to google this article.
So I think this issue can crop up in some unexpected places.