A nurse who visited a Toronto SARS-affected hospital came down with SARS-like symptoms, shortly followed by one of her two children. As a result, many people at her work and everyone at her child's daycare and a nearby school were quarantined.
At one point, over 1,000 people in the town of 6,000 were in quarantine!
Several days into the quarantine, tests showed that she was not affected by SARS at all (I guess they positively identified another pathogen) and they let everyone out.