txt1 <-
  paste("#' # A reprex for knitr::spin comments",
        "#'",
        "#' The check for start (/*) and end (*/) comments can fail to identify",
        "#' notable issues.  Consider a document with the first occurance of comment to",
        "#' be the *end* comment and the last comment to be an *start* comment.",
        "#'",
        "# */",
        "#' This text is in an 'comment' section but the characters denoting the comment",
        "#' are backwards. This will not be in the .Rmd file, even though the start and",
        "#' end delimiters are backwards",
        "# /*",
        "#'",
        "#' This doc can be spun to a .Rmd",
        sep = "\n")

x1 <- knitr::spin(text = txt1, knit = FALSE, format = "Rmd")
cat(x1, sep = "\n")
#> # A reprex for knitr::spin comments
#> 
#> The check for start (/*) and end (*/) comments can fail to identify
#> notable issues.  Consider a document with the first occurance of comment to
#> be the *end* comment and the last comment to be an *start* comment.
#> 
#> 
#> This doc can be spun to a .Rmd

txt2 <-
  paste("#' # A reprex for knitr::spin comments",
        "#'",
        "# */",
        "#' This text is in an 'comment' section but the characters denoting the comment",
        "#' are backwards. This will not be in the .Rmd file.",
        "# /*",
        "#'",
        "#' A little R code",
        "2 + 2",
        "#'",
        "# */",
        "#' Another comment",
        "# /*",
        "#' This doc can be spun to a .Rmd, but the comment start and end delimators are",
        "#' interchangeable.",
        sep = "\n")

x2 <- knitr::spin(text = txt2, knit = FALSE, format = "Rmd")
cat(x2, sep = "\n")
#> # A reprex for knitr::spin comments
#> 
#> 
#> A little R code
#> 
#> ```{r }
#> 2 + 2
#> ```
#> 
#> 
#> This doc can be spun to a .Rmd, but the comment start and end delimators are
#> interchangeable.

txt3 <-
  paste("#' # A reprex for knitr::spin comments",
        "#'",
        "#' text ...",
        "#'",
        "# */ outside comment",
        "#",
        "# ... code that is commented out during dev work, perhaps,",
        "#",
        "# */",
        "# A comment that is a comment and will be here for the long term.",
        "# /*",
        "#",
        "# ... code and text, next line ends outside comment.",
        "# /*",
        "#'",
        "#' document text",
        sep = "\n")

x3 <- knitr::spin(text = txt3, knit = FALSE, format = "Rmd")
#> Error in knitr::spin(text = txt3, knit = FALSE, format = "Rmd"): comments must be put in pairs of start and end delimiters

xfun::session_info("knitr")
#> R version 3.6.2 (2019-12-12)
#> Platform: x86_64-apple-darwin15.6.0 (64-bit)
#> Running under: macOS Catalina 10.15.2
#> 
#> Locale: en_US.UTF-8 / en_US.UTF-8 / en_US.UTF-8 / C / en_US.UTF-8 / en_US.UTF-8
#> 
#> Package version:
#>   evaluate_0.14   glue_1.3.1      graphics_3.6.2  grDevices_3.6.2
#>   highr_0.8       knitr_1.27.2    magrittr_1.5    markdown_1.1   
#>   methods_3.6.2   mime_0.8        stats_3.6.2     stringi_1.4.5  
#>   stringr_1.4.0   tools_3.6.2     utils_3.6.2     xfun_0.12      
#>   yaml_2.2.0