There is a weird Google bug with the cache feature. It seems to only impact home pages. For example, a search for this site in Google returns a Google search result of the home page (we aren't penalized yet):
But when you click on the cache link, it takes you to a dead page:
This does not appear to impact inside pages, but rather only the root page (aka home page). Why?
I should note that this isn't uncommon and I wouldn't worry about this impacting your Google rankings.