I know of accidents where pilots messed up published IMC let-downs and others where pilots attempted to let down without any sort of pre-planned procedure. However, I can only think of one accident where an "unpublished" let down resulted in an accident and that occurred because the procedure wasn't followed at all accurately. Paradoxically, and sadly, a CAA Ops inspector was on board and perished along with the crew.
Indeed if you are referring to G-HAUG, the primary cause was the deviation from the planned procedure. In the conclusions:
3.1.20 No evidence was found that would indicate that the aircraft would have experienced any difficulty in following the selected route, MOIRA - WARRN - MAP - B, if the aircraft had been operated throughout in the fully coupled mode, with Nav Capture, and allowed to fly the selected route without manual intervention.
There is an argument that a PANS-OPS compliant procedure would have offered more margin for error. But in general, deviating more than 1.3 miles from an RNP 0.3 final approach segment of a published procedure is also bad news.