What should I do if I find that I am under-dressed or dressed
inappropriately at a wedding?

Take off any inappropriate clothing immediately!

No, seriously: if the wedding invitation specified any form of dress,
there is no excuse for not being appropriately attired  (and it should
if anything out of the ordinary is expected:   black tie or Jamaican
beachwear.  If it didn't, the sefe ground is a formal suit for men and
equivalent dress for women.  No problem if you are overdressed.  IF
black tie was expected or is worn by others, a dark suit can still
pass.  If you turn up in a sleaveless T-shirt, then disappear
immediately, go change, buy something, forego the wedding and start
drafting your letter of apology.
It is an affront to the couple to attend inappropriately attired.  If
you have any doubts, talk to someone who will know.

Find a trusted person or two among the guests (or among the groomsmen
or bridesmaids) and ask.  If a beloved aunt and uncle arrive to find
that everyone else is more formally dressed than they had anticipated,
the rest of the family--including the newlyweds--should think it more
important to have the aunt and uncle present.

If you are a social acquaintance of the bride or groom and realize too
late that the invitation said "black tie," leave the gift and slink
away.

If the wedding is in a church with a balcony, maybe someone can sneak
you into the balcony so you can see without being a spectacle
yourself.

There are lots of situations in-between, but the basics are (1) the
relationship between the underdressed guest and the bride, groom, or
families, and (2) the degree of underdressing.

I see no real crisis. If it happens, it happens. Remain in the
background and excuse yourself early. It's better to have attended
under attired than to have missed the event altogether.

tutuzdad-ga