Probable cause of bad signature?

[Nicholas Cole]

I've just noticed a curious signature on my own key - apparently one
that I made myself a few years ago (2004), but which --check-sigs is
now listing as "bad".  It is the only signature on the key showing as
bad.  It probably doesn't matter at all, but I'm still curious to know
what might have caused it.  Does anyone have any ideas?  The signature
is class 10.  All of the other self-sigs seem to be fine.

Best wishes,

Nicholas