What is the term used when someone is acting of their own accord outside of reasonable or advised bahaviour or instruction?  I don't mean volenti non fit injuria theres is a another simular term it might not even be a latin thing or a defence thing its more of a we told them not to do it and they are adults so if they do it its of their own accord and not in lines with the cordinated activity or reason why they are their,  I am pretty sure theres a word for that. But I may well be going off my little rocker!

Proprio Motu

A Frolic of ones own means something slightly different, a frolic is when the employee is acting on his own and for his own benefit, rather than a minor sidetrack in the course of obeying an order from the employer so you would have to prove a case and be significantly persuasive in doing so.  It can still attract vicarious libility https://www.newlawjournal.co.uk/content/new-landmark-vicarious-liability

Proprio Motu relates to an official action or act that is taken on ones own initiative or without the request of another which again has a slightly different meaning the the OP describes.

Do you mean carrying out an authorised action in an unauthorised way? in the law of neglegence we normaly talk about the level of standards that a "reasonable person" would exersise in a similar situation.  Defence would be "contributory neglegence" where we would build a case to determine that the failure of an injured party to act prudently was considered to be a contributory factor in the injury which they have suffered

Edited by user 13 September 2018 10:14:16(UTC)  | Reason: Spelling

I was going to suggest one of the below:

breach, contravene, flout, disobey, derogate from, 

But then found this which I think defines what you are looking for? 

https://www.dundaslawyers.com.au/can-an-employer-sack-an-employee-for-not-following-directions/

Relates/defines the terms of serious misconduct and unreasonable refusal.