Incorrect conversion from R Markdown to LaTeX - latex

Question

Why does the following R Markdown minimal (non)-working example not compile to PDF?
---
header-includes:
- \usepackage{fancyhdr}
- \pagestyle{fancy}
- \lhead{}
- \chead{}
- \rhead{The performance of new graduates}
- \lfoot{From: K. Grant}
- \cfoot{To: Dean A. Smith}
output:
pdf_document:
keep_tex: yes
latex_engine: xelatex
---
# Test
In particular, the problematic conversion happens to -\lfoot{From: K. Grant} and -\cfoot{To: Dean A. Smith}, as seen in the output .tex file:
\usepackage{fancyhdr}
\pagestyle{fancy}
\lhead{}
\chead{}
\rhead{The performance of new graduates}
true
true
For some reason, both of these lines are converted to true causing
LaTeX error: Missing \begin{document}
thereby preventing the document from compiling to PDF.
Changing \lfoot and \cfoot to just about anything else seems to lead to them being converted correctly. So what's going on here? I take it that there must be a problem with either knitr or pandoc in the conversion process.
NB: I'm not too familiar with R Markdown, and this is a follow-up question to Headers and footers created in Fancyhead not shown in PDF on TeX.SX on Tom's behalf.

Answer 1

The : character is the problem. pandoc seems to be trying to parse the header-includes content as if it were variables, and : is used to separate variables and values. It compiles if you quote the lines in question (don't forget, then, to escape the leading backslash)
---
header-includes:
- \usepackage{fancyhdr}
- \pagestyle{fancy}
- \lhead{}
- \chead{}
- \rhead{The performance of new graduates}
- "\\lfoot{From: K. Grant}"
- "\\cfoot{To: Dean A. Smith}"
output:
pdf_document:
keep_tex: yes
latex_engine: xelatex
---
# Test

Answer 2

Stitching the answer here with trial and error, I found that only by deleting the title key and using YAML pipe-denoted multiline string syntax would it compile:
---
output:
pdf_document:
keep_tex: yes
latex_engine: xelatex
header-includes: |
\usepackage{fancyhdr}
\pagestyle{fancy}
\fancyhead{}
\fancyfoot{}
\lhead{My Title}
\rhead{My Note}
\lfoot{\today}\rfoot{Page \thepage}
---

Answer 3

I recommend using pandoc's raw_attribute feature (enabled by default) to mark the raw LaTeX section as such. This is the most robust solution.
header-includes: |
```{=latex}
\usepackage{fancyhdr}
\pagestyle{fancy}
\fancyhead{}
\fancyfoot{}
\lhead{My Title}
\rhead{My Note}
\lfoot{\today}\rfoot{Page \thepage}
```