I made an interesting discovery while verifying streets near my neighborhood in San Diego. There was about a mile's worth of streets charted in one little area that simply didn't appear in Yahoo's aerial imagery, so I investigated them in person. Turns out they really *aren't* there. I placed a marker to mark the imaginary intersection of Buchanan and Oklahoma Streets, roughly the center of the place where they all appeared before I corrected the map.
In reality, the phantom "streets" traced the contours of little valleys in the canyon by the neighborhood. Some of the streets just extended existing streets far beyond their real length, others (such as the aforementioned Buchanan and Oklahoma) seem to have been made up entirely.
Where did those streets come from? Not sure. But I learned a little more about the history of the data, and that may lend itself to some guesses. I found San Diego's GIS data, and it shows the same phantom streets that were in the TIGER data--so it seems to follow that the TIGER data came from the City of San Diego. A historical note accompanying the data mentions that the it was originally compiled by the local gas and electric utility (SDG&E), then handed over to the city government in 1990.
Maybe the utility company invented those streets to watch out for anyone leaking their data? Or, as my wife suggests, maybe the streets were planned streets that never came into being? or roads that once existed but are gone now? I guess I will have to look up some older maps at the library to solve the mystery.