Really the bug is more severe than just failing (and innocent) xscreensaver.
backtrace:
#0  0x00002ade19df2582 in _mesa_x86_64_transform_points4_perspective () from /usr/lib/dri/i915_dri.so
#1  0x00002ade19d85909 in ?? () from /usr/lib/dri/i915_dri.so
#2  0x00002ade19d7bd7b in _tnl_run_pipeline () from /usr/lib/dri/i915_dri.so
#3  0x00002ade19d7c2a4 in _tnl_draw_prims () from /usr/lib/dri/i915_dri.so
#4  0x00002ade19d7454e in vbo_exec_vtx_flush () from /usr/lib/dri/i915_dri.so
#5  0x00002ade19d700fd in vbo_exec_FlushVertices () from /usr/lib/dri/i915_dri.so
#6  0x00002ade19cfbe2e in _mesa_Flush () from /usr/lib/dri/i915_dri.so
#7  0x000000000040486a in draw_polytopes (mi=0x7fff93e3fd30) at polytopes.c:3064
#8  0x000000000040738b in xlockmore_screenhack (dpy=0x63fb30, window=73400322, want_writable_colors=<value optimized out>, 
    want_uniform_colors=0, want_smooth_colors=0, want_bright_colors=0, event_mask=66, hack_init=0x404ec0 <init_polytopes>, 
    hack_draw=0x4046d0 <draw_polytopes>, hack_reshape=0x404e70 <reshape_polytopes>, hack_handle_events=0x404ce0 <polytopes_handle_event>, 
    hack_free=0) at xlockmore.c:444
#9  0x0000000000404fc8 in screenhack (dpy=0x6c75c0, window=6922368) at ./../xlockmore.h:158
#10 0x0000000000405a81 in main (argc=1, argv=0x7fff93e404c8) at ./../screenhack.c:679
#11 0x00002ade18810b44 in __libc_start_main () from /lib/libc.so.6
#12 0x0000000000403199 in _start ()

BUT! the point is not a backtrace, but a code inside driver:
(gdb) disassemble 
Dump of assembler code for function _mesa_x86_64_transform_points4_perspective:
0x00002ade19df2530 <_mesa_x86_64_transform_points4_perspective+0>:      mov    0x10(%rdx),%ecx
0x00002ade19df2533 <_mesa_x86_64_transform_points4_perspective+3>:      movzbl 0x14(%rdx),%eax
0x00002ade19df2537 <_mesa_x86_64_transform_points4_perspective+7>:      mov    %ecx,0x10(%rdi)
0x00002ade19df253a <_mesa_x86_64_transform_points4_perspective+10>:     movl   $0x4,0x18(%rdi)
0x00002ade19df2541 <_mesa_x86_64_transform_points4_perspective+17>:     orl    $0xf,0x1c(%rdi)
0x00002ade19df2545 <_mesa_x86_64_transform_points4_perspective+21>:     test   %ecx,%ecx
0x00002ade19df2547 <_mesa_x86_64_transform_points4_perspective+23>:     xchg   %ax,%ax
0x00002ade19df254a <_mesa_x86_64_transform_points4_perspective+26>:     je     0x2ade19df25b3 <_mesa_x86_64_transform_points4_perspective+131>
0x00002ade19df254c <_mesa_x86_64_transform_points4_perspective+28>:     mov    0x8(%rdx),%rdx
0x00002ade19df2550 <_mesa_x86_64_transform_points4_perspective+32>:     mov    0x8(%rdi),%rdi
0x00002ade19df2554 <_mesa_x86_64_transform_points4_perspective+36>:     movd   (%rsi),%mm0
0x00002ade19df2557 <_mesa_x86_64_transform_points4_perspective+39>:     pxor   %mm7,%mm7
0x00002ade19df255a <_mesa_x86_64_transform_points4_perspective+42>:     punpckldq 0x14(%rsi),%mm0
0x00002ade19df255e <_mesa_x86_64_transform_points4_perspective+46>:     movq   0x20(%rsi),%mm2
0x00002ade19df2562 <_mesa_x86_64_transform_points4_perspective+50>:     prefetch (%rdx)
0x00002ade19df2565 <_mesa_x86_64_transform_points4_perspective+53>:     movd   0x28(%rsi),%mm1
0x00002ade19df2569 <_mesa_x86_64_transform_points4_perspective+57>:     xchg   %ax,%ax
0x00002ade19df256c <_mesa_x86_64_transform_points4_perspective+60>:     punpckldq 0x38(%rsi),%mm1
0x00002ade19df2570 <_mesa_x86_64_transform_points4_perspective+64>:     prefetchw 0x20(%rdi)
0x00002ade19df2574 <_mesa_x86_64_transform_points4_perspective+68>:     movq   (%rdx),%mm4
0x00002ade19df2577 <_mesa_x86_64_transform_points4_perspective+71>:     movq   0x8(%rdx),%mm5
0x00002ade19df257b <_mesa_x86_64_transform_points4_perspective+75>:     movd   0x8(%rdx),%mm3
0x00002ade19df257f <_mesa_x86_64_transform_points4_perspective+79>:     movq   %mm5,%mm6
0x00002ade19df2582 <_mesa_x86_64_transform_points4_perspective+82>:     pfmul  %mm0,%mm4
0x00002ade19df2586 <_mesa_x86_64_transform_points4_perspective+86>:     punpckldq %mm5,%mm5
0x00002ade19df2589 <_mesa_x86_64_transform_points4_perspective+89>:     pfmul  %mm2,%mm5
0x00002ade19df258d <_mesa_x86_64_transform_points4_perspective+93>:     pfsubr %mm7,%mm3
0x00002ade19df2591 <_mesa_x86_64_transform_points4_perspective+97>:     pfmul  %mm1,%mm6
0x00002ade19df2595 <_mesa_x86_64_transform_points4_perspective+101>:    pfadd  %mm4,%mm5
0x00002ade19df2599 <_mesa_x86_64_transform_points4_perspective+105>:    pfacc  %mm3,%mm6
0x00002ade19df259d <_mesa_x86_64_transform_points4_perspective+109>:    movq   %mm5,(%rdi)
0x00002ade19df25a0 <_mesa_x86_64_transform_points4_perspective+112>:    add    %rax,%rdx
0x00002ade19df25a3 <_mesa_x86_64_transform_points4_perspective+115>:    movq   %mm6,0x8(%rdi)
0x00002ade19df25a7 <_mesa_x86_64_transform_points4_perspective+119>:    add    $0x10,%rdi
0x00002ade19df25ab <_mesa_x86_64_transform_points4_perspective+123>:    dec    %ecx
0x00002ade19df25ad <_mesa_x86_64_transform_points4_perspective+125>:    prefetch 0x20(%rdx)
0x00002ade19df25b1 <_mesa_x86_64_transform_points4_perspective+129>:    jne    0x2ade19df2570 <_mesa_x86_64_transform_points4_perspective+64>
0x00002ade19df25b3 <_mesa_x86_64_transform_points4_perspective+131>:    femms  
0x00002ade19df25b5 <_mesa_x86_64_transform_points4_perspective+133>:    retq   
0x00002ade19df25b6 <_mesa_x86_64_transform_points4_perspective+134>:    nopw   %cs:0x0(%rax,%rax,1)
End of assembler dump.

We see that SIGILL fires at the first pfmul
pfmul is AMD 3-D Now opcode, and  it fails since I have Intel Core 2 Duo CPU.
So the bug can manifest itself in any app with OpenGL, this could possibly lock up the screen, is this enough for High priority?