The rig crew was reaming back to bottom after making a connection. The hole fill was encountered before the kelly bushings reached the table, so the driller rotated the string with the kelly spinner to clean out the fill. While lining up the table and kelly bushings, the mud pump pressure spiked and simultaneously the mud pump “pop-off” (pressure relief) valve blew. At the same instant the stand pipe pressure gauge blew out of the standpipe. The pressure gauge glanced off the “A-leg” and struck a worker who was cleaning the breakout tong dies. The worker received a laceration to the head, three broken ribs, a fractured shoulder, and a collapsed lung.