There are cases where significant damage to lenses, such as major front-element scratches, have been shown to imperceptibly affect image quality. This has led me to wonder: Are there any known cases where lens damage, such as scratches or fungus, have led to an improvement in image quality?
The usual metrics for image quality may be considered: Sharpness, Resolution, Contrast, Chromatic Abberation, Field Curvature, Color Accuracy, etc. Since improvements with respect to one metric often involve trade-offs in others, it would be reasonable for answers to describe what aspect of image quality is improved.
Image quality is not about how pleasing an image appears, which I would refer to as image character. For instance, a low contrast lens has low image quality in terms of contrast, but may produce an image with pleasing "character". Many film-era lenses have this "problem.
Non-damage alterations: The addition of air-glass interfaces to a lens has the potential to reduce sharpness, contrast, resolution, color accuracy, etc. However, doing so does not necessarily negatively affect those metrics. Rather, additional lens elements are necessary to increase image-quality as measured by various metrics.
Non-photographic scenario: A guitar a friend of mine used to own. It had a large 3cm hole in it that was unintentionally produced when someone dropped or hit it. Despite, or perhaps ¿because of?, that hole, he said it was the best sounding guitar he had owned. He was referring both to the character of the sound as well as more objective sound qualities such as the ability of the guitar to project sound.
Suppose we measured the amount of resonance produced by the guitar before and after punching the hole. Like scratches to lenses, most holes in guitars probably don't affect resonance much. Too many holes, or holes in the wrong place (like scratches on a rear element), and resonance would worsen. But happen to punch a hole in the right place, and resonance would increase.
Hypothetical scenario: Suppose I were to tinker with a lens and happened to damage the aperture blades so that the aperture were to be irregularly shaped. I might later notice that the lens had become sharper (as measured by photographing resolution charts). However, I might also be unhappy with its effect on bokeh (character).