Though no longer listed among Mozart's posthumous works in the Neue Mozart Ausgabe (New Mozart Edition), "Der Spiegelkanon" ("the mirror canon") is in a contemporary style to W.A. Mozart. [was once apparently III/10.16]
Nonetheless it is a fun set of canonic violin duets that belong to a type of canon sometimes known as Tafelmusik (table muaic), though this designation invites confusion with baroque pieces played at feasts, banquets and dinner gatherings in the courts of Europe.
With these mirror canons duets, the page is placed on a table between the two musicians, and each plays from the opposite end of the page the same music. Thus each player, from the other's perspective is playing the music upside down. Each staff of music is therefore adorned with a clef on the left side as well as an upside-down clef on the right.
Here is the music of Number 1 viewed both ways:
from the score at the International Music Score Library Project
Of course, once scholars have satisfied themselves that the piece is not by Mozart after all, it is another matter entirely to disabuse the rest of the world of that fact. So you will often still find these canons attributed to Mozart.