When using the multi interface over HTTP and the server returns a Location
  header, the running easy handle will get stuck in the CURLM_STATE_PERFORM
  state, leaving the external event loop stuck waiting for data from the
  ingoing socket (when using the curl_multi_socket_action stuff). While this
  bug was pretty hard to find, it seems to require only a one-line fix. The
  break statement on line 1374 in multi.c caused the function to skip the call
  to multistate().

  How to reproduce this bug? Well, that's another question.  evhiperfifo.c in
  the examples directory chokes on this bug only _sometimes_, probably
  depending on how fast the URLs are added. One way of testing the bug out is
  writing to hiper.fifo from more than one source at the same time.