Disclaimer

A disclaimer is traditionally what one adds to the beginning of one's fanfic. It usually runs along the lines of 'I do not own [continuum] nor any of the characters therein' and then generally what passes for a joke, often about [author] turning in their grave and how much power it's generating. This joke is so prevalent that it led to the formation of the Department of Dead Author Electricity Generation.

As fanfiction hovers in a grey sort of area of legality that people like to avoid talking about, a disclaimer is a polite nod to those who actually own a continuum. It is considered courteous to include one, and actually making it amusing gets you bonus points with readers.

Authors of a few quarantined continua have chosen to ban fanfiction entirely, and a disclaimer does nothing to either penetrate the creativity shield enclosing these worlds or mitigate the rudeness of writing about characters whose author has specifically excluded from any stories but his/her own.

Fanfiction in continua old enough to be in the public domain does not run into these copyright issues, but a disclaimer is still considered to be polite, a way to acknowledge the author's original work.

Disclaimers are a courtesy, not a legal requirement, so 'lack of a disclaimer' is not a charge. Claiming to own a continuum IS a charge, even if done in an attempt to be 'funny'.