I always thought the joke in Best in Show about a Starbucks being across the street from another Starbucks was a bit of an exaggeration. Then I took a stroll down Mission Street, between First and the Ferry Building. After I passed by the 3rd or 4th one, I noticed they are so densely set, you can see at least one more Starbucks from any Starbucks you're passing by. Their store locator doesn't make it as clear (even considering the small scale on the map), but it's really shocking.
UPDATE: Bill points out that the Starbucks across the street from another Starbucks actually does exist in Vancouver (note the addresses), and that's where Christopher Guest noticed them during the filming. It all makes sense now.