Sysop vigilantiism is a common phenomena on large public wikis. People who are unwisely granted sysop status and priveleges abuse them to harass, annoy, lie about, or block IPs of people they don't like, even though those people didn't break any rules. Most typically it is a person unqualified to be a moderator who received the powers to carry out some technical function, who begins to treat others as if they were there to satisfy their own petty desires or answer to their own petty concerns.
It is closely related to, but a different phenomenon than, sysop vandalism. Often the two occur together, e.g. vigilantiism to block IPs followed by destruction of the useful material they contributed, even though most other users consider it valuable.