Leica recently discontinued its replacement service for M9 cameras afflicted by the widespread "sensor corrosion" issue that surfaced back in 2014. Now a new report claims that it wasn't "damaged" coatings that caused the corrosion, but the fact that protective coatings were inexplicably left out completely.
Leica no longer has access to the CCD image sensor inside the Leica M9 series of cameras. As a result "defective sensors cannot be replaced" moving forward, and you'll need to upgrade to an M10 or another CMOS-based Leica camera if this happens to you.
Leica is acknowledging that there is a "sensor corrosion" issue that potentially affects photographers who use its M-series digital rangefinders. The problem shows up as white spots in photographs that cannot be removed by a traditional cleaning of the sensor.