Strong form through the first three rounds propelled Webby and Sitah into the final 34 in the battle for the medals but six dropped rails put paid to any chance of a top-10 finish.
The exertions of three energy-sapping rounds over challenging courses in the most humid of conditions seemed to finally catch up with the 13-year-old mare. A light preparation after Sitah confounded experts with a speedy recovery from stomach surgery in time to line up as the reserve combination for the Games, eventually led to an anti-climatic finish for the pair.
"Towards the middle of the course, she just got a bit flat and a bit lower and started mowing a couple down," Webby said.
"She finished off all right but I think she had had enough and was just going through the motions a bit."
The United States-based Webby, who grew up in Taranaki, made the most of the lifeline offered by the injury-enforced withdrawal of Daniel Meech and Sorbas.
Last night it all unraveled for Webby, 37, as Sitah couldn't reproduce the solid jumping efforts of the previous rounds. He said it was probably the worst jumping performance of her career.
The 24 penalties left the combination at the tail of the 34-strong field, with the next worst being on 16 faults.
"I feel bad for the horse really, she's given her guts for the last three or four days," Webby said.
"I think the whole week just caught up with her tonight, it was pretty hot and sometimes it just doesn't go your way.
"It would have been nice to end a bit better than that but that's showjumping."
In a thrilling finish, a two-way jump-off was required to find the gold medalist while seven competed for the bronze.
In the end Eric Lamaze and Hickstead secured the gold medal for Canada ahead of Sweden's Rolf-Goran Bengtsson (Ninja) and Beezie Madden (Authentic) of the US.
Officials managed to get the final of the equestrian events safely completed ahead of the threatened arrival of Typhoon Nuri. But there was no way to avoid the storm caused by a doping scandal, revealed just prior to the showjumping finale.
The view held by many experts that Sha Tin had provided the best facilities and best competition in Olympic equestrian history counted for little when the sport was left reeling by the news of four positive dope tests.