I man in San Franscisco heard a ruckus out on the street and went to investigate. The source of the noise was two men with an angle grinder going at a bike lock. Obviously cutting a U-lock in an angle grinder is suspect, so he went to go investigate.
After being confronted, and giving the normal excuses, the thief on the right takes off on the bike. The guy follows on foot and calls the police. The thief eventually ditches the bike and continues on an attempted escape.
Unfortunately, the police could not charge him with theft without the presence of the rightful owner, but he was jailed for existing warrants.