Mozart’s famous piece Ein kleine Nachtmusik has four movements – but in his personal catalogue, Mozart recorded five.
You’ve heard Eine kleine Nachtmusik, I am sure of it. It is perhaps Mozart’s most famous piece of music, played everywhere to the point of cliché, but if you need a reminder here it is:
It was not always so famous. The piece may never have been performed in Mozart’s lifetime, and did not even appear in print until well after his death. This may go some way towards explaining the mystery at its heart.
Eine kleine Nachtmusik has four movements: Allegro, Romance, Minuet and Trio, and Finale. However, Mozart’s own records tell a different story. In his personal catalogue, Mozart recorded this piece as follows:
A little serenade, consisting of an allegro, a minuet and trio, a romance, a minuet and trio, and a finale. For two violins, viola, and bass instruments.
Eine kleine Nachtmusik
So, in Mozart’s hand, we have evidence of a missing movement: the first minuet and trio, supposedly appearing between the opening allegro and the romance. No such minuet appears in Mozart’s manuscripts, and we are left to wonder about what has been lost.
That, of course, has not stopped academics from searching for the missing movement. One possible candidate is the Piano Sonata in B-flat major. The minuet in this sonata was once attributed to Mozart, but we now know that it was composed by a contemporary of Mozart named August Eberhard Müller. Müller reworked some of Mozart’s other pieces into his own work, so it is hypothesized that this minuet may actually be Müller’s arrangement of Mozart’s missing movement.
Rather than trying to find the lost music, others have tried to write something Mozart-esque to go in its place. I quite like Debbie Wiseman’s Ein Bisschen Fehlt (“a bit missing”), which you can hear in the last link below.
The Generalist Academy 
I’d imagine most are.