When such a false positive arrives you could click "the spam button"
again (its tooltip should say "mark as not junk") this should fix the
problem.
I have been doing that for some time now and it still marks them as
spam. It is not always the same people but certain ones do get picked
on a lot. It seems to get it by subject too I guess.
Any ideas where this info is kept? I thought of unmerging and then
cleaning out the directory and emerging it again. I'm not sure that
will work either though.