Why would one wing stall before the other? Reading about the Douglas C-133, I see this:


  The second issue discovered was stall characteristics gave very little
  warning to the crew. The left wing was found to stall before the right
  wing. The fix was simple, a small strip of metal was attached to the
  right wing causing it to stall at the same time the left wing would
  stall.


The issue affects the whole type. What would cause one wing to reliably stall before the other?
In general, aircraft aerodynamics can be affected from manufacturing tolerances or consistent asymmetries. 

The reason can be due to any asymmetry in the wings or fuselage shapes, or wing-to-fuselage fixtures.