PageSpeed uses serf to fetch things over http. Sometimes serf gives errors back to PageSpeed that get reported in error messages:

  Serf status 120171(APR does not understand this error code)

The serf documentation is limited, however, so it's hard to look these up. So here's a reference, from serf.h in serf 1.3.8:

code	meaning
120101	This code is for when this is the last response on this connection: i.e. do not send any more requests on this connection or expect any more responses.
120102	This code is for when the connection terminated before the request could be processed on the other side.
120103	This code is for when the connection is blocked—we can not proceed until something happens - generally due to SSL negotiation-like behavior where a write() is blocked until a read() is processed.
120104	This code is for when something went wrong during deflating compressed data e.g. a CRC error.
120105	This code is for when a response received from a http server is not in http-compliant syntax.
120106	The server sent less data than what was announced.
120107	The proxy server returned an error while setting up the SSL tunnel.
120108	The server unexpectedly closed the connection prematurely.
120170	SSL certificates related errors.
120171	SSL communications related errors.
120190	General authentication related errors.
120191	None of the available authentication mechanisms for the request are supported.
120192	Authentication was requested by the server but the header lacked some attribute.
120193	Authentication handler initialization related errors.
120199	Error code reserved for use in the test suite.

Why are these hard to look up in the code? They're all defined with macros:

  #define SERF_ERROR_RANGE 100
  #define SERF_ERROR_START \
    (APR_OS_START_USERERR + \
     SERF_ERROR_RANGE)

  #define SERF_ERROR_CLOSING \
    (SERF_ERROR_START + 1)
  #define SERF_ERROR_REQUEST_LOST \
    (SERF_ERROR_START + 2)
  ...

This means that a search for [120108] or something won't turn anything up, which is annoying. This also means that on some platforms serf error codes can have a different prefix. In PageSpeed, though, this is all determined at compile time because it includes its dependencies, so it should be the same everywhere.