Thanks. Yes, I guess I realised it had something to do with ambiguous
grammars, but I hadn't really considered that my input was a valid table
(mea culpa).

So I thought aha! I'll change the terminating "---" to "..." (in the hope
of an error), but this of course gives the output shown below.

I wonder whether there could be a warning in the manual (or maybe there is
and I've missed it?) and/or perhaps a suggestion that it might be a good
idea to avoid complex YAML metadata blocks (in favour of metadata files)?
I'd be happy to create an issue and/or a MANUAL.md pull request.

% cat bad.md
---
history:
- number: Release 1.2
  changes: |
  - TBD
...

% pandoc bad.md
<hr />
<p>history: - number: Release 1.2 changes: | - TBD …</p>


On Wed, 2 Jun 2021 at 17:47, John MacFarlane <jgm-TVLZxgkOlNX2fBVCVOL8/A@public.gmane.org> wrote:

>
> Well, this actually IS a valid "simple table".  So how is
> pandoc to know that you intended it as a YAML metadata block?
> Pandoc tries first to parse things as YAML blocks, and then as
> tables, if either works, the parse succeeds with no error.
>
> William Lupton <wlupton-QSt+ys/nuMyEUIsrzH9SikB+6BGkLq7r@public.gmane.org> writes:
>
> > Should I expect syntax errors in YAML blocks to be detected and reported?
> >
> > This has a syntax error in the YAML block:
> >
> > % cat bad.md
> > ---
> > history:
> > - number: Release 1.2
> >   changes: |
> >   - TBD
> > ---
> >
> > This fixes it:
> >
> > % diff bad.md good.md
> > 5c5
> > <   - TBD
> > ---
> >>     - TBD
> >
> > The latest pandoc:
> >
> > % ~/Downloads/pandoc-2.14.0.1/bin/pandoc -v
> > pandoc 2.14.0.1
> > ...
> >
> > Pandoc thinks the bad YAML is a table:
> >
> > % ~/Downloads/pandoc-2.14.0.1/bin/pandoc bad.md
> > <table>
> > <tbody>
> > <tr class="odd">
> > <td>history:</td>
> > </tr>
> > <tr class="even">
> > <td>- number: Release 1.2</td>
> > </tr>
> > <tr class="odd">
> > <td>changes: |</td>
> > </tr>
> > <tr class="even">
> > <td>- TBD</td>
> > </tr>
> > </tbody>
> > </table>
> >
> > But it's OK with the good YAML:
> >
> > % ~/Downloads/pandoc-2.14.0.1/bin/pandoc good.md
> >
> > Parsing the file as YAML reports the error:
> >
> > % ~/Downloads/pandoc-2.14.0.1/bin/pandoc --metadata-file bad.md
> > Error at "bad.md" (line 5, column 0):
> > Unexpected ' '
> >
> > % ~/Downloads/pandoc-2.14.0.1/bin/pandoc --metadata-file good.md
> > ^C
> >
> > --
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> .
>

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