title: Knitting error when knitting html on RStudio
body: <p>I've been a regular R user for 1.5 years. Never seen this happen and have no idea how to fix it. I've read the whole Internet in search of a solution without luck. I have tried restarting my computer and reinstalling RStudio. I am able to knit any .Rmd as a .pdf or .html the first time I try it, but subsequent tries fail with the error messages below. This happens with both tried-and-true .Rmd that have worked previously as well as new template .Rmd files created through RStudio's File-->New option. If I re-install RStudio, it allows knitting of any .Rmd file again on the first try. </p>
  
  <pre><code>Loading required package: knitr
  Warning message:
  package ‘knitr’ was built under R version 3.2.3 
  
  processing file: HW16.Rmd
    |.................................................................| 100%
    ordinary text without R code
  
  
  output file: HW16.md
  
  [1] "HW16.md"
  Error generating HTML preview for ~/Dropbox/MyProjects/spring2016/advancedMachineLearning/HW16.Rmd r error 4 (R code execution error)
  </code></pre>
  
  <p>The console generates:
  <a href="https://i.stack.imgur.com/dnaW7.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/dnaW7.png" alt="enter image description here"></a></p>