Another older untracked piglit regression. Fails at least on i965 (GM45) and swrast, first bad commit: 

commit f3db20da1a04fd64732c62631d59aac801a1a59d
Author: Kenneth Graunke <kenneth@whitecape.org>
Date:   Fri Nov 16 14:52:01 2012 -0800

    mesa: Rework crazy error code rules in glDrawBuffers().
    
    Perhaps most importantly, this patch adds comments quoting the relevant
    spec paragraphs above each error condition.
    
    It also makes three changes:
    - For FBOs, GL_COLOR_ATTACHMENTm where m >= MaxDrawBuffers is supposed
      to generate INVALID_OPERATION (not INVALID_ENUM).
    - Constants that refer to multiple buffers (such as FRONT, BACK, LEFT,
      RIGHT, and FRONT_AND_BACK) are supposed to generate INVALID_OPERATION,
      not INVALID_ENUM.
    - In ES 3.0, for FBOs, buffers[i] must be NONE or GL_COLOR_ATTACHMENTi
      or else INVALID_OPERATION occurs.  (This is a new restriction.)
    
    Fixes es3conform's draw-buffers-api test.
    
    v2: The error path was missing a "return" like all the other error
    paths.  Also, we may as well call it glDrawBuffers in the error message
    since the ARB suffix doesn't exist in ES 3.
    
    Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
    Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

This wasn't probably spotted before, because at the time this was introduced, the failing piglit test wasn't written yet. However it showed up in my recent piglit comparison of last 5 mesa releases.

./bin/gl-3.2-draw-buffers-errors -auto -fbo
Mesa: User error: GL_INVALID_OPERATION in glDrawBuffersARB(unsupported buffer)
Mesa: User error: GL_INVALID_OPERATION in glDrawBuffersARB(unsupported buffer)
Mesa: User error: GL_INVALID_OPERATION in glDrawBuffersARB(buffer)
Expected GL_INVALID_ENUM with GL_FRONT but received: GL_INVALID_OPERATION
Mesa: User error: GL_INVALID_OPERATION in glDrawBuffersARB(buffer)
Expected GL_INVALID_ENUM with GL_BACK but received: GL_INVALID_OPERATION
Mesa: User error: GL_INVALID_OPERATION in glDrawBuffersARB(buffer)
Expected GL_INVALID_ENUM with GL_LEFT but received: GL_INVALID_OPERATION
Mesa: User error: GL_INVALID_OPERATION in glDrawBuffersARB(buffer)
Expected GL_INVALID_ENUM with GL_RIGHT but received: GL_INVALID_OPERATION
Mesa: User error: GL_INVALID_OPERATION in glDrawBuffersARB(buffer)
Expected GL_INVALID_ENUM with GL_FRONT_AND_BACK but received: GL_INVALID_OPERATION
PIGLIT: {'result': 'fail' }