Yes, I hope someday the parse_intermixed_args patch can be released... but I know it is not relevant to this issue.

I was aware of the %(substitution_variables) in the default help formatter, but I (1) goofed and entered % without escaping it (2) was surprised at how unhelpful the Traceback was at isolating the problem.

Happily, my code had only a few instances of %) so I was able to isolate it fairly quickly, but the error report certainly shows up at quite a distance (execution-wise) from the location of the source bug.

I haven't looked at the source for the HelpFormatter code: if it concatenates all the help text and then does substitutions en masse, then it would be difficult to isolate the error to a particular argument. If, on the other hand, it loops through the help text for each argument, doing the substitutions, and later formatting and concatenating, then surrounding the substitution attempt with a try: block so that the name of the argument with the faulty help text could be reported, that would be a big help to this situation, at little cost.