Systematic error in reflection experiment

 This is how it should be. The angle of incidence from the normal line (90 degrees) is 50 degrees and so is the angle of reflection.

Here the mirror has not been set on its base line correctly. The incident ray is still where it was but the angle of reflection is 56 degrees. This is a SYSTEMATIC ERROR because every single reading made from now on will be 6 degrees out for the same reason.

Note that the mirror is only 3 degrees out along the base line. That moves the normal line by 3 degrees increasing the angle of incidence to 53 degrees although the incident ray looks as though it is in the same place. Hence the angle of reflection also increases by 3 degrees. 3 degrees on each side of the normal means the line seems to have moved by 6 degrees in total.