On BXT I can reliably reproduce crashes for ext_framebuffer_multisample.accuracy by running the tests repeatedly.  I can not produce the crash before the following commit:

18f688d62a69501072d6783bbce34d1a755ef2c9
Author:     Ilia Mirkin <imirkin@alum.mit.edu>
AuthorDate: Sun Jan 31 01:09:40 2016 -0500
Commit:     Ilia Mirkin <imirkin@alum.mit.edu>
CommitDate: Tue Feb 2 17:08:46 2016 -0500

    mesa: use default geometry's samples when there are no attachments
    
    Whether multisampling is turned on depends, in part, on whether
    attachments are themselves multisample surfaces. However when there are
    no attachments, we should rely on the default geometry for this.
    
    Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
    Reviewed-by: Marek Olšák <marek.olsak@amd.com>

The crash crashes consistently in update_program, as with the following trace:
#0  0x00007f99ba7b3740 in update_program (ctx=0x7f99c0818040) at main/state.c:287
#1  _mesa_update_state_locked (ctx=ctx@entry=0x7f99c0818040) at main/state.c:473
#2  0x00007f99ba7b3ae1 in _mesa_update_state (ctx=ctx@entry=0x7f99c0818040) at main/state.c:504
#3  0x00007f99baa0c9c8 in brw_draw_rectlist (brw=brw@entry=0x7f99c0818040, rect=rect@entry=0x7ffe0c97a5a0, num_instances=num_instances@entry=1) at brw_meta_fast_clear.c:203
#4  0x00007f99baa0d928 in brw_meta_resolve_color (brw=0x7f99c0818040, mt=0x1a7f990) at brw_meta_fast_clear.c:879
#5  0x00007f99baa51a45 in intel_miptree_resolve_color (brw=brw@entry=0x7f99c0818040, mt=<optimized out>) at intel_mipmap_tree.c:2005
#6  0x00007f99ba9fefc4 in intel_update_state (ctx=0x7f99c0818040, new_state=<optimized out>) at brw_context.c:198
#7  0x00007f99ba7b32ed in _mesa_update_state_locked (ctx=ctx@entry=0x7f99c0818040) at main/state.c:493
#8  0x00007f99ba7b3ae1 in _mesa_update_state (ctx=0x7f99c0818040) at main/state.c:504
#9  0x00007f99ba6e9e55 in _mesa_valid_to_render (ctx=ctx@entry=0x7f99c0818040, where=where@entry=0x7f99baaf453d "glDrawElements") at main/context.c:1947
#10 0x00007f99ba6d06a1 in check_valid_to_render (ctx=0x7f99c0818040, function=0x7f99baaf453d "glDrawElements") at main/api_validate.c:44
#11 0x00007f99ba6d0b7d in validate_DrawElements_common (ctx=ctx@entry=0x7f99c0818040, mode=mode@entry=4, count=count@entry=6, type=5125, indices=0x0, caller=caller@entry=0x7f99baaf453d "glDrawElements") at main/api_validate.c:412
#12 0x00007f99ba6d0e90 in _mesa_validate_DrawElements (ctx=ctx@entry=0x7f99c0818040, mode=mode@entry=4, count=count@entry=6, type=type@entry=5125, indices=indices@entry=0x0) at main/api_validate.c:438
#13 0x00007f99ba8141a5 in vbo_exec_DrawElements (mode=4, count=6, type=5125, indices=0x0) at vbo/vbo_exec_array.c:1125
#14 0x000000000040309d in DownsampleProg::run (this=0x1a7f20c, src_fbo=0x1a7ef78, dest_width=64, dest_height=64, srgb=false) at ../tests/spec/ext_framebuffer_multisample/common.cpp:220
#15 0x00000000004036fa in Test::downsample_color (this=0x1a7eba0, downsampled_width=64, downsampled_height=64) at ../tests/spec/ext_framebuffer_multisample/common.cpp:347
#16 0x0000000000403c74 in Test::draw_reference_image (this=0x1a7eba0) at ../tests/spec/ext_framebuffer_multisample/common.cpp:475
#17 0x00000000004041ee in Test::run (this=0x1a7eba0) at ../tests/spec/ext_framebuffer_multisample/common.cpp:594
#18 0x0000000000404bcc in test_create_and_execute () at ../tests/spec/ext_framebuffer_multisample/accuracy.cpp:151
#19 0x0000000000404c7c in piglit_display () at ../tests/spec/ext_framebuffer_multisample/accuracy.cpp:167
#20 0x00007f99c03caba5 in run_test (gl_fw=0x1775c20, argc=5, argv=0x7ffe0c97aaa8) at ../tests/util/piglit-framework-gl/piglit_fbo_framework.c:52
#21 0x00007f99c03b1117 in piglit_gl_test_run (argc=5, argv=0x7ffe0c97aaa8, config=0x7ffe0c97a970) at ../tests/util/piglit-framework-gl.c:199
#22 0x0000000000404861 in main (argc=5, argv=0x7ffe0c97aaa8) at ../tests/spec/ext_framebuffer_multisample/accuracy.cpp:55