Neil Brown wrote:

When writing to the array, the new parity block will sometimes be
calculated assuming that it was previously correct.  If all updates to
a particular parity block are of this sort, then it will still be
incorrect when you lose a drive, and data recovered based on that
parity block will be incorrect.

Until you lose a drive, it will have no visible effect.

There is a slight chance that this happened to me recently - would a
  echo check > /sys/block/mdX/md/sync_action
detect this?
and would
  echo repair > /sys/block/mdX/md/sync_action
fix it?

David