Arbitrary classification systems

In reality, all topical classification systems, as well as alphabetical systems are arbitrary. From the history of the alphabet, we see that its formation could give no absolute logic to its order. Topical classifications, although regarded as definitive by encyclopedias and tree structured organization systems, can also become arbitrary on large scales.
Some interesting classification systems, good and bad:
(these hopefully will be separate entries linked later)
Classification and reclassification during apartheid.
The meaning of planets and stars.
Periodic table.
Dewey Decimal
Amazon