The Combination Harvester was invented by Ned Simnel during the summer that Bill Door was working at Miss Renata Flitworth's farm. This mechanical contrivance was to be the dawning of a new technological age on the Disc. It was a reaper of corn that horrified Bill Door utterly. He tried to beat it with his method of reaping: one stalk at a time. He nearly did so too, but mortals couldn't. As always, people were dismissive, but were beginning to come round to the idea. Unfortunately the Combination Harvester came to grief when Bill Door needed the tarpaulin from around it during a very fierce storm, and simultaneously removed a small but vital part.
Has parallels with the tank created by Urn during the events of Small Gods. Perhaps it's a good job Urn and Ned never met, especially as Ned had ideas in which the horse would be outdated and mechanical objects could move under their own motive power...
Clearly based on the Combine Harvester on Roundworld. The combine harvester, or simply combine, is a machine that harvests grain crops. It combines into a single operation what previously had taken three separate operations (reaping, binding, and threshing). It was invented in 1838. Originally they were drawn by mule or horse teams and used a bull wheel to provide power. Later, steam power was used, and George Stockton Berry integrated the combine with a steam engine using straw to heat the boiler.
Death trying to outdo the Combination Harvester is a neat metaphor for how the reaper cares for us as individuals. Non Timetis Messor, remember.