I know you qualified it as "in theory", yet I find myself thinking of wash-trading as an example where transactional value is neutral or even very slightly negative, because they actually value manipulating the metrics.
In that case, you could argue that "true GDP" didn't change at all, merely our measurement of it had error. But pretty soon you ask 'well, what is the real signal?' And it's turtles all the way down.
Again, economics is weird, but perhaps interesting because it's weird. (I'll confess I didn't care about the field at all until it suddenly got very interesting in ~2008.)
