manifested by failing to use the system in a proper way.
Personally, I think that is more the issue, based on inadequate knowledge of the system functions. This accident (
https://aviation-safety.net/wikibase/72613) in 1985 wasn't even using the upper modes, it was the TRE trying to demonstrate the inherent heading hold function of the ATT mode, by lifting into the hover with feet away from the micro-switches on the pedals. Problem was, they had already been released on a different heading, and the aircraft turned without them being depressed again. So, as it lifted, it turned onto the memorised heading.......as it was designed to.