[AUTHOR’S NOTE: Edited 2021-10-23 to reflect changes in reality]
I will add more information later, but I have created a theme (Hugo Minimal Test Theme) which is designed to make it easy to add in minimal changes to create samples of problems that crop up, using a less complex theme than a full-blown one.
https://github.com/danielfdickinson/hugo-minimal-test has the theme and
I’ve also created a demo website for the theme[AUTHOR’S NOTE: removed site].
Note that this has been updated from the First version of Hugo Minimal Test Theme which as been archived and is no longer maintained.
Another interesting tool that was suggested to the author, and which he has built into the theme (as well as making it available as a module for others who wish to use it) is a set of Hugo Debug Tables which one can view on the debug-tables demo website. The tables provide a comprehensive set if information about the variables that are in play when using the Hugo site generator, on every page or via shortcode, depending on one’s preference.
Note that this has been updated from the First version of Hugo Debug Tables which as been archived and is no longer maintained.