I sometimes to deal with this unwanted practice with a little humor. I
tell the students that I attribute any errors in addition, subtraction,
etc. to be of human origin, and that by telling me they are human
errors they are insulting my intelligence. I go on to tell them that
they should report any bovine, canine or feline errors as such.
Many seem to get the point. A few report bovine errors trying to be
funny.