I was trying to install an electric receptacle inside our pantry (there is an existing receptacle on the opposite side of the same wall). I used a magnetic stud finder and determined where the studs were, and selected a spot inside the same stud cavity (several inches to the side, so the receptacle boxes had some room between them, but some inches below the normal 12-16 inch from the floor; I chose some 6-10 inches from the floor for convenience).
After cutting the drywall, I hit wood on both sides of the rectangle. I also inserted a nail a bit above and hit wood in two different places. Picture:
That wall is supporting the higher of two flights of stairs (the pantry is under that flight), so I guess it could be a plywood shear wall, but I'm not sure. The house was built in 2013.
I've since patched it up with compound, but I'm still curious as to what I might have hit in that wall. Any clues?