MySQL failing to start with an "unknown variable" error

Symptoms

The MySQL service is failing to start with an error about an "unknown variable":

[Server] unknown variable '“THE_UNKNOWN_VARIABLE”'.
[Server] Aborting

Description 

This issue is caused by an unknown variable(s) within the MySQL configuration file /etc/my.cnf.

Workaround

Remove, correct, or comment out the unknown variable(s) specified in the MySQL error log in the /etc/my.cnf file.