Sigma sent out a notice that the very earliest recipients of its well-regarded 28-70 f/2.8 zoom may lose its ghosting protection over time. This isn’t referencing the spiritual health of the lens, but rather likely the nano-scale lens surface coatings that physically interact with lightwaves to prevent light from moving in certain angles, preventing “ghosting” effects on the sensor.
It is also theoretically possible that a non-glass lens element was used whose material’s refractive index turned out to not be as permanent as had been anticipated, although this is much less likely. That lens has three of its 18 elements – the asphericals – that are not specifically referred to as glass.
Lenses made with serial numbers prior to 55488834 are affected.