If call glProgramStringARB with string:
"!!ARBfp1.0\nOUTPUT tempout = result.color;\nTEX  tempout.rb, fragment.texcoord.yxzw, texture, 1D;\n\nEND\n";

Mesa will report below info:
Mesa 6.5.3 implementation error: i915_program_error: Can't (yet) swizzle TEX arguments
Please report at bugzilla.freedesktop.org

I used 3D driver 915_dri.so, if use software rendering, above error info is still reported.
In i965 platform, Mesa does not report above error info.

The backtrace info is:
#0  translate_program (p=0x8685e48) at i915_fragprog.c:763
#1  0xb7c07427 in i915ValidateFragmentProgram (i915=0x8399830)
    at i915_fragprog.c:983
#2  0xb7c0e903 in i915_render_start (intel=0x8399830) at i915_vtbl.c:50
#3  0xb7c25797 in intelRenderStart (ctx=0x8399830) at intel_tris.c:769
#4  0xb7c1d4c5 in intel_run_render (ctx=0x8399830, stage=0x83e3074)
    at intel_render.c:212
#5  0xb7cba709 in _tnl_run_pipeline (ctx=0x8399830) at tnl/t_pipeline.c:158
#6  0xb7c2eaad in intelRunPipeline (ctx=0x8399830) at intel_tris.c:764
#7  0xb7cbb368 in _tnl_draw_prims (ctx=0x8399830, arrays=0x83d1288, 
    prim=0x83cfde4, nr_prims=1, ib=0x0, min_index=0, max_index=3)
    at tnl/t_draw.c:403
#8  0xb7cb41ae in vbo_exec_vtx_flush (exec=0x83cfcc0)
    at vbo/vbo_exec_draw.c:215
#9  0xb7cafe20 in vbo_exec_FlushVertices (ctx=0x8399830, flags=1)
    at vbo/vbo_exec_api.c:680
#10 0xb7d4b804 in _mesa_ReadPixels (x=0, y=0, width=4, height=1, format=6407, 
    type=5126, pixels=0x82731c0) at main/drawpix.c:277
#11 0xb7e857ef in glReadPixels (x=0, y=0, width=4, height=1, format=6407, 
    type=5126, pixels=0x82731c0) at ../../../src/mesa/glapi/