title: How do I understand the warnings from rbind?
body: <p>If I have two data.frames with the same column names, I can use rbind to make a single data frame.  However, if I have one is a factor and the other is an int, I get a warning like this:</p>
  
  <blockquote>
    <p>Warning message: In <code>[&lt;-.factor</code>(<code>*tmp*</code>, ri, value = c(1L, 1L, 0L,
    0L, 0L, 1L, 1L,  :   invalid factor level, NA generated</p>
  </blockquote>
  
  <p>The following is a simplification of the problem:</p>
  
  <pre><code>t1 &lt;- structure(list(test = structure(c(1L, 1L, 2L, 1L, 1L, 1L, 1L, 
  1L, 1L, 2L), .Label = c("False", "True"), class = "factor")), .Names = "test", row.names = c(NA, 
  -10L), class = "data.frame")
  t2 &lt;- structure(list(test = c(1L, 1L, 0L, 0L, 0L, 1L, 1L, 1L, 1L, 1L
  )), .Names = "test", row.names = c(NA, -10L), class = "data.frame")
  rbind(t1, t2)
  </code></pre>
  
  <p>With the single column, this is easy to understand, but when it is part of a dozen or more factors, it can be difficult.  What is there about the warning message to tell me which column to look at?  Barring that, what is a good technique to understand which column is in error?</p>