I’d heard of a problem with an early M9 a while back, but recently there have been several fora across the Internet where users were reporting that they were seeing abnormal areas in their images when taken at small apertures. Most often, these areas appear as white, the opposite as does dust or oil spots. Leica uses Schott glass containing the IR block over the sensor on the recent M cameras, and it now appears that the filter layer on the glass may be getting damaged on some cameras, possibly even by sensor cleaning. The reason the spots appear as white is probably because the underlying sensor is sensitive to near IR, so once the block is removed over an area, the sensor is collecting far more light than usual.
Leica has identified the problem, which occurred on M9, M9-P, M Monochrom and M-E cameras (it does not affect the M Type 240), and is working on a permanent solution. In the meantime, Leica will apparently offer a free replacement of the sensor cover glass for any cameras that exhibit the issue. They also offer to check your camera for the problem, though you need to do this by appointment through your local distributor or Leica Customer Care. Should you be considering an upgrade to the Type 240, Leica will also be willing to examine your current M and make you a trade-in offer. Finally, anyone that’s been charged for this sort of repair in the past is eligible for a refund from Leica. Leica’s offers are regardless of whether the camera is still in warranty or not.
As a side note, most IR/AA filtration these days uses Lithium Niobate or another similar crystal type. When it degrades, you can often start to see the crystal structure imposed in your pixels. There were a few Nikon DSLRs I’ve seen over the years where the filter degraded and needed to be replaced.