A few unusual specimens come to mind.
A friend has a green glass beer bottle with a slipper limpet (Crepidula
fornicata) inside.  The shell is about 2.5 inches (6.25 cm) in length and
almost 2 inches (5 cm) in width.  The bottle opening is about 3/4 of an inch
(1.9 cm) in diameter.  Apparently the larval limpet swam into the bottle and
settled on the inner surface, then grew to maturity there, attached to the
glass.  The bottle must have served as its private greenhouse, growing algae
to serve as its food.
Then there was the "bivalve chain" I had a few years ago - a large Lima lima
vulgaris, to which was attached a large Chama brassica, to which was
attached a medium-size thorny oyster (Spondylus variegatus), to which was
attached a medium-size cock's-comb oyster (Lopha cristagalli), to which was
attached another, smaller, Chama brassica.  The whole thing curved around
into sort of a horseshoe shape.  Too bad it didn't remain in the ocean long
enough for some other bivalve to fill in the gap and make a complete ring!
Xenophora (carrier shells) are always a good source of the unusual.  You
never know what they will attach to themselves.  I have seen two specimens
with land snails attached.  One specimen attached a piece of wood about 2
inches in length, with a nail sticking through it.  One had a bottle cap
attached.  One carrier attached an auger (Terebra) shell, the other end of
which was attached to a large chunk of another carrier shell.  But my
favorite was the specimen with the built-in collection data.  A Xenophora
pallidula from the Philippines had attached a triangular piece of brown
glass, with the single word "Philippines" printed across it in white.  The
source of this carrier costume component was later identified as a San
Miguel Beer bottle.
Regards,
Paul M.