In my first time through, playing solo, I drew the Hunter and the Driver as my investigators. The Hunter plowed along, wiping out cultists and shoggoths (atlhough she did go insane for a while); the Driver tagged along behind her until he collected enough town cards, then raced off to seal the next Gate. I won handily enough--but then I realized that I'd missed "if a site has three cultists, do not place a fourth; instead, an Awakening Ritual occurs."
So I tried again. In the second game, using a Hunter and a Detective, I'd gotten two gates closed when my Hunter successfully fought a shoggoth--but the Sanity roll you take for fighting a shoggoth triggered another two cultists appearing, on a location which already had two, meant Dagon Awakened, which added one cultist to every Gate location, which triggered another Awakening, and thens plummted out of control and then Cthulhu.