title: Why does `log_slow_queries` break `my.cnf`?
body: <p>Why can't I use slow_query_log on MySQL 5.6 on CentOS 6.4?</p>
  
  <pre><code>[mysqld]
  datadir=/var/lib/mysql
  socket=/var/lib/mysql/mysql.sock
  ...
  
  ## Logging
  
  ## *** THESE LOGS WORK JUST FINE ***
  log_error                       = /var/log/mysql/error.log
  general_log_file                = /var/log/mysql/mysql.log
  general_log                     = 1
  
  ## *** THESE LOGS BREAK MYSQL ***
  #log_slow_queries               = /var/log/mysql/slow.log
  #long_query_time                = 5
  #log-queries-not-using-indexes
  </code></pre>
  
  <p>Here's the <code>/var/log/mysql</code> directory:</p>
  
  <pre><code>$ ls -lh
  total 100K
  -rw-r----- 1 mysql root   47K Nov 22 06:02 error.log
  -rw-rw---- 1 mysql root   42K Nov 22 06:05 mysql.log
  -rw-rw---- 1 mysql mysql    0 Nov 22 06:01 slow.log
  </code></pre>
  
  <p>If I uncomment the log_slow_query lines in the <code>/etc/my.cnf</code> I receive the following error:</p>
  
  <pre><code>$ /etc/init.d/mysql restart
  Shutting down MySQL.. SUCCESS!
  Starting MySQL..... ERROR! The server quit without updating PID file (/var/lib/mysql/server.domain.com.pid).
  </code></pre>
  
  <p>What am I missing?</p>