Background
The flight was delayed by over 30 minutes due to repeated delays. During this time, a mechanic informed the captain that a serviceable part on his aircraft needed to be swapped with a part from another aircraft. The captain stated that he was angry and his blood pressure was 'up to the stroke level'.
Sequence of Events
Minutes later, the mechanic told the captain that maintenance control had decided against changing the parts and that the aircraft was 'all closed up and ready to go.' The captain understood this to mean that the aircraft doors were closed, while the mechanic intended to convey that all maintenance access doors were secured. Neither the captain nor the first officer checked the door status lights on the overhead panel or the EICAS display. Both were unaware that a gate agent was still onboard attempting to resolve an oversold passenger situation. At that time, the L2 door was open and the jet bridge was still attached to the aircraft.
The captain then asked the tug driver if they were ready to go. According to the captain, he heard the tug driver say that all doors and access panels were closed. The tug driver, however, stated that he replied that all service doors and cargo doors, except for the jet bridge, were secured and locked. The captain told the tug driver they were clear to pushback. The tug driver signaled the wing walkers and began the pushback, during which the aircraft struck the jet bridge, damaging it, the L2 door, a section of aircraft skin, and the L2 evacuation slide.
Aftermath
After the accident, the right wing walker reported that he had not given the tug driver the all-clear signal required by company policy prior to pushback. The left wing walker assumed that something was wrong with the jet bridge since the captain had ordered a pushback and signaled the driver that the ramp was clear.