> Martin, your reproducable behaviour involves consistently disabling 
> functionality that those threads you mention may rely on

Right. So with the current implementation, they notice immediately that
something is wrong.

> (and since 
> this i is a global state that no one messes with, they are unlikely to 
> be doing so)

Except that Python messes with it, so in the small window where the
assertions get disabled, they may run past them