A construction worker fell to his death yesterday evening at the Ansonia, at 438 12th Street in Park Slope. The construction worker, Henryk Siebor, stepped onto a scaffold that gave way and fell, and two other workers were left dangling in the air on harnesses. Both mechanical failure and human error played a role in the accident, authorities said. The workers have been replacing bricks on the facade of the building, a former clock tower, for three months. A Department of Buildings spokesperson said a stop-work order had been issued for the site and citations were expected following an investigation.