title: data.table is not filtering correctly
body: <p>While using data.table and filtering I am getting wrong results.</p>
  
  <p>I am exposing here a reproducible example to see the problem:</p>
  
  <pre><code>set.seed(123)
  results &lt;- data.table(id = 1:1000000, v1 = rnorm(1000000))
  rs &lt;- data.table(type = 1:1000000 %% 5, prob = rnorm(1000000))
  
  results[, `:=`(cls = rs$type, cls.prob = rs$prob)]
  results[cls != 0 &amp; abs(v1 - cls.prob) &lt; 0.3, `:=`(cls = 3, cls.prob = 1)]
  summary(results[cls == 4, cls])
     # Min. 1st Qu.  Median    Mean 3rd Qu.    Max. 
     #    4       4       4       4       4       4 
  results[cls != 0 &amp; abs(v1 - cls.prob) &lt; 0.2, `:=`(cls = 3, cls.prob = 1)]
  summary(results[cls == 4, cls])
     # Min. 1st Qu.  Median    Mean 3rd Qu.    Max. 
     #    4       4       4       4       4       4 
  results[cls != 0 &amp; abs(v1 - cls.prob) &lt; 0.5, `:=`(cls = 3, cls.prob = 1)]
  summary(results[cls == 4, cls])
     # Min. 1st Qu.  Median    Mean 3rd Qu.    Max. 
     # 3.00    4.00    4.00    3.87    4.00    4.00 
  </code></pre>
  
  <p>As you can see the last summary should ouput 4 in every slot.</p>
  
  <p>Does anyone know what is going on?</p>
  
  <p>I am working on this version of R </p>
  
  <pre><code>&gt;version
                 _                           
  platform       x86_64-suse-linux-gnu       
  arch           x86_64                      
  os             linux-gnu                   
  system         x86_64, linux-gnu           
  status                                     
  major          3                           
  minor          1.3                         
  year           2015                        
  month          03                          
  day            09                          
  svn rev        67962                       
  language       R                           
  version.string R version 3.1.3 (2015-03-09)
  nickname       Smooth Sidewalk 
  </code></pre>
  
  <p>and this version of data.table:</p>
  
  <pre><code>&gt; packageVersion('data.table')
  [1] ‘1.9.4’
  </code></pre>