Re: Starcraft 2: 8 Medivac drop
Its hard dealing with drops with spines because terran drops have such a high mobility, ie he just drops somewhere else and attacks at a angle. And a couple of spines are nothing to 3/3 infantry with stim.
That said I usually put a spine or two and a spore to prevent drops behind the mineral line to discourage drops there. So if they come they won't get stuck behind minerals and the lings can get to them.
I also try and keep a pack of 8-10 mutas alive late game to intercept drops.
Late late game I can see mass crawlers in your base when you are getting broodlords as a means to free up supply.