From Wikipedia:"The enigmatic Canon 1 à 2 from J. S. Bachs Musical Offering (1747), The manuscript depicts a single musical sequence that is to be played front to back and back to front."
A crab canon (also known by the Latin form of the name, canon cancrizans; as well as retrograde canon, canon per recte et retro or canon per rectus et inversus) is an arrangement of two musical lines that are complementary and backward, similar to a palindrome. Originally it is a musical term for a kind of canon in which one line is reversed in time from the other (e.g. FABACEAE played simultaneously with EAECABAF). A famous example is found in J. S. Bach's The Musical Offering, which also contains a canon ("Quaerendo invenietis") combining retrogression with inversion, i.e., the music is turned upside down by one player, which is a table canon.