When I worked for Intel in 1980, they were working on a CPU called the
   IAPX 432, which as I recall had the intended feature that they were
   installed in pairs, one called a "generator" and the other called a
   "checker".

   https://en.wikipedia.org/wiki/Intel_iAPX_432?wprov=sfla1

   The latter verified that the "generator" had done its job correctly.

   On Mon, Jun 7, 2021 at 6:01 PM, Karl
   <gmkarl@gmail.com> wrote:

   Summary:
   Google is collecting evidence indicating that at least around 3% of
   modern cups are silently corrupting data in ways that users don't
   notice.

   A pathological cpu core is described that performs AES in some way such
   that it can reliably decrypt its own encrypted documents, but no other
   discovered cores can do so.
   The failures are called "corrupt execution errors" and the invisibly
   failing cores "mercurial" .  Blamed on problems of miniaturization.
   Chip manufacturers have not spoken yet.  Google is finding more
   problems as they improve their software for detecting them.
   Google's paper linked from the article appears to be
   [1]https://sigops.org/s/conferences/hotos/2021/papers/hotos21-s01-hochs
   child.pdf .  I have not visited the link myself.

References

   1. https://sigops.org/s/conferences/hotos/2021/papers/hotos21-s01-hochschild.pdf