Mirroring introduces phantom angles

When a part is mirrored using the ‘Mirror’ option in the symmetry menu, there is a possibility that the rotation information is not applied corectly to the mirrored part. Normaly this corruption is very minor, as in around 10^-5°, but here it is in the 10^-2° range, which can noticably affect the dynamics of a craft. For example the plane in the pictures has a noticable tendency to roll right. Reseting the rotations on one wing by typing in the rotation menu from the root of the wing outwards can temporarily correct the errors, but they appear again, after the craft is shifted in the reference frame by for example saving, which also introduces small errors in the positions of parts.
What might be happening is that all those tiny errors in position and rotation add up.