I finally made a mistake that I like, and I'd like to find out what
the mistake was so I can reproduce it!

I've been using X on various Unices for twenty years without ever 
understanding how the font-rendering systems really work (shame on
me).

My 'mistake' is this:  I somehow changed the default font of
(at least) two programs to comic-sans-ms.  This is one of my
favorite fonts, so I'm sure I did something deliberately to
pick that font -- but I have no idea what I did, or when, or
how.

The two programs I'm referring to are xpat2(solitaire game) and
xfontsel.  There may be other programs also, but I haven't noticed
them yet.

Where should I start looking for font changes that affect more than
one program?

I tried fc-list, which did not show the 'comic' font at all. What next?

Thanks for any clues.