Scrubbing is supposed to detect and repair all errors in the pool. However, it
wrongly ignores active spare devices. The problem can easily be reproduced in
OpenZFS at git rev 0ef125d with these commands:

truncate -s 64m /tmp/a /tmp/b /tmp/c
sudo zpool create testpool mirror /tmp/a /tmp/b spare /tmp/c
sudo zpool replace testpool /tmp/a /tmp/c
/bin/dd if=/dev/zero bs=1024k count=63 oseek=1 conv=notrunc of=/tmp/c
sync
sudo zpool scrub testpool
zpool status testpool # Will show 0 errors, which is wrong
sudo zpool offline testpool /tmp/a
sudo zpool scrub testpool
zpool status testpool # Will show errors on /tmp/c, which should've already been fixed

FreeBSD head is partially affected: the first scrub will detect some errors,
but the second scrub will detect more.