The X server dies with signal 11 when I run the rectangle-rounding-error test
from the cairo test suite using the XCB backend. I've tested the Debian-packaged
version 1:1.0.2-9 with both the XFree86 DDX (with XAA) and Xvfb, and both
demonstrate the problem. I haven't had a chance to test with a newer server yet,
or a server with debugging symbols.

Several X requests are being batched together in the packet that kills the X
server, and XCB doesn't (yet) have an XSynchronize equivalent, so I can only say
for sure that it's one of these requests at this point:
CreatePicture, FillRectangles, SetPictureTransform, ChangePicture,
SetPictureFilter, Trapezoids, FreePicture, CreatePixmap, FreePixmap, GetImage

My poorly-educated guess from looking at the backtraces is that it's a bug in
the fb layer. The backtrace from my regular X server follows; the Xvfb trace is
less useful since I don't have debugging symbols, but also segfaults in the fb
layer.

Backtrace:
0: /usr/bin/X(xf86SigHandler+0x89) [0x80b6849]
1: [0xffffe420]
2: /usr/lib/xorg/modules/libfb.so(fbCompositeGeneral+0x830) [0xa7777fd0]
3: /usr/lib/xorg/modules/libfb.so(fbComposite+0x1d6) [0xa7780d76]
4: /usr/lib/xorg/modules/libxaa.so(XAAComposite+0x1ee) [0xa773a3de]
5: /usr/bin/X [0x8153e28]
6: /usr/bin/X [0x8150816]
7: /usr/bin/X(CompositePicture+0xeb) [0x813e41b]
8: /usr/bin/X(miTrapezoids+0x233) [0x813d893]
9: /usr/bin/X [0x815413e]
10: /usr/bin/X(CompositeTrapezoids+0x93) [0x813e5f3]
11: /usr/bin/X [0x8146e14]
12: /usr/bin/X [0x81414a5]
13: /usr/bin/X(Dispatch+0x19b) [0x8086b8b]
14: /usr/bin/X(main+0x48e) [0x806e5de]
15: /lib/tls/libc.so.6(__libc_start_main+0xc8) [0xa7cf9ea8]
16: /usr/bin/X(FontFileCompleteXLFD+0xad) [0x806d911]

I'll attach the full X server log and the wireshark packet capture.