SAN FRANCISCO — SFGate reports that the San Francisco Bay Area’s Clipper smart cards have a built-in mechanism that allows some riders to exit BART stations even if their cards lack sufficient value to pay full fare. It is a customer convenience intentionally built into the system, officials say. However, after some media gave a tutorial in how to abuse the card, transit officials are watching closely to see whether riders are abusing the system. For the full story, click here.