I was actually considering a more in-depth explanation of sub shells would be a good topic for another post. Because along with () there is also $() and `` and <() and stuff | otherstuff that all end up invoking sub shells... and while I'm at it, here-docs might fit in there, too, as you can do stuff like bash << EOF bash <<< stuff...

I almost always forgot to popd after I pushd... I generally end up cd -ing...