Astigmatism
One of the clearest examples of differences in out-of-focus images. The left and right image always will look 90° rotated. Otherwise the images are the same. Again collimation might clear this problem completely.

1/4 wave astigmatism in an unobstructed telescope

The left image is generated for a perfect 6” refractor and the right the same telescope with 0.5 wave Astigmatism (X/Y).