The jaws on this wrench are not parallel:

http://home.comcast.net/~bobengelhardt/wrench.jpg

Specifically, the face of the movable jaw is perpendicular to the
"ways", but the fixed face is not. But they aren't sprung or worn that
way. It seems that it was made like that. But why? Just plain poor
quality? But that's *really* bad. It's an old wrench, not Chiwandian.
It says drop forged, if that helps.

Bob