And if you ignore the warning?
My car if I forget to close the boot or do not close the boot properly (or a kid does not shut the door properly) gives me a warning.
The dashboard is designed very intelligently with lots of warning lights, but they are all OFF and do not clutter my vision or give rise to confusion.
A warning light coming ON is clearly visible in such well designed and conceived Human Interface system designed to reduce the Human Error of missing the warning (if a warning light comes ON, it is hard to miss).
So,
a. on the assumption the MKVI Display/HUD gives you a warning (visual and audible and vibrating) that you forgot to install the scrubber and
b. the MKVI Human Interface is NOT cluttered with information where the warning could be missed
then, if the diver ignores the warning, it is User Error number 2.
The first human error was to forget to insert the scrubber pre-dive.
The second human error would be to fail to notice and/or take action after the MKVI issues a warning.
The above assumes the absence of stress.
Add stress, and we see less, hear less, and miss important cues from the environment.
It is not uncommon for person under stress to become so focused in the task at hand (i.e. landing a plane on an aircraft carrier) that they see what they expect to see and miss all warnings as if they never happened.
They are just not perceived.
Add stress, and we get fear.
If fear is used to "focus," fear enhances our ability to deal with stress.
Otherwise, we start tearing around or trashing through the brush... unable to perform correct tasks despite best training... even remove the regulator and drown (if we have difficulty breathing as in a CO2 hit it is an instinctive reaction to clear the mouth from whatever you have in the mouth to breathe).
So, the answer to your question is complex.
BUT, it all starts with the equipment being properly designed to take into account of Human Factors.
THEN, you need to understand and be trained in Situational Awareness (not trained in "Human Factors").
The training methodology is key to foster Situational Awareness (you won't get it by reading a book, watching a video, listening to a lecture... it requires a special type of training method).
Far too easy to dismiss any accident due to Human Factor.
There will often always be a Human Error to blame or a "violation" (i.e. "the pilot did not sleep enough the night before"), when that is just one of the many factors at play.