Abstract: At approximately 2:00 p.m. on August 29, 2007, Employee #1, of Concannon Construction, Inc., was carrying a piece of plywood at a construction site. He encountered a 108 in. tall wood-framed wall composed only of vertical 2 by 4 studs, erected 16 in. on center. He walked through a 13.5 in. gap between two of the studs and stepped into an 8 ft by 10 ft floor opening that was covered by a piece of plywood leftover after the removal of some temporary shoring, apparently giving the impression of being a walking surface. As Employee #1 stepped on the plywood, it gave way and he fell 20 ft onto a concrete surface. He was transported by paramedics to San Francisco General Hospital, where he was treated for ten days. The floor opening was guarded on two sides only; the other sides were surrounded by the wood-framed stud walls. There was no horizontal guardrail on the wall studs to prevent walking through the gaps. The floor opening was not actually covered, and the plywood was neither properly secured nor braced. The employer was cited for a violation, serious, of T8CCR 1632(b), for the uncovered floor opening, and a violation, serious of T8CCR 1632(h), for the lack of a guardrail around the opening.