Pandoc error parsing BibTeX file

I get the following error so often (and I always forget how to solve it) that I thought I'd post a solution.

pandoc-citeproc: "stdin" (line 2421, column 2):
unexpected "m"
expecting "c", "C", "p", "P", "s" or "S"
pandoc: Error running filter /usr/lib/rstudio/bin/pandoc/pandoc-citeproc
Filter returned error status 1
Error: pandoc document conversion failed with error 83
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