Back in the days of MapQuest there was a (usually) very good site called mapsonus.com, but it evidently had one of the ferries that crossed Boston Harbor in its map as a zero distance link.

Since it was offline, the bug was obvious although a bit frustrating - you had to put in multiple waypoints to make it forget its urge to send you on the ferry to Hull when you were trying to get to parts of the South Shore.