John White, in his interesting list of borrowings, included:

>(5) The finales of Haydn's Symphony No 13 in D and Mozart's Symphony No 41
>in C both share the same 4 note phrase which, in each case, is treated
>contrapuntally.

This one has been the subject of some speculation.  The four-note phrase
of the Jupiter is something like a motto for Mozart: it appears in a number
of his works, going all the way back to the Symphony No. 1, and is treated
contrapuntally several times.  It's been suggested that he first saw
Haydn's 13th Symphony as a boy while in Paris.

I love Paul MacKay's comparison of the March of the Priests in Die
Zauberflote with the Canadian national anthem.  If you listen to enough
Mozart, you discover that he wrote a lot of Christmas carols, too.
Cherubino's aria "Voi che sapete" is "Adeste Fidelis," the slow movement of
an obscure wind divertimento is very clearly "Silent Night," and the slow
movement of the Piano Concerto No. 14 sounds like "Hark, the Herald Angels
Sing" (which Mendelssohn actually did write, and you wonder if he had the
Mozart in mind).

Peter