With my xinerama setup xorg-server-18.104.22.1682 I get a segfault when I move the mouse to the left of the main screen.
I'm not sure what information is needed to diagnose this issue so please ask for what is relevant.
Steps to Reproduce:
Without any backtraces, that's going to be hard to fix. Please read this guide  and try to get a proper backtrace out of Xorg. Those will be available in /var/log/Xorg.0.log.
Once you get a nice backtrace, please attach the full log here.
I'll do that ASAP. Probably tomorrow.
Program received signal SIGSEGV, Segmentation fault.
miDCSaveUnderCursor (pDev=0x26877c0, pScreen=0x2388d50, x=<value optimized out>, y=<value optimized out>, w=26, h=32) at midispcur.c:536
536 midispcur.c: No such file or directory.
#0 miDCSaveUnderCursor (pDev=0x26877c0, pScreen=0x2388d50, x=<value optimized out>, y=<value optimized out>, w=26, h=32) at midispcur.c:536
#1 0x00000000005529e5 in miSpriteSaveUnderCursor (pDev=0x26877c0, pScreen=0x2388d50) at misprite.c:1039
#2 0x0000000000552c97 in miSpriteSetCursor (pDev=0x26877c0, pScreen=0x2388d50, pCursor=0x26eee00, x=<value optimized out>,
y=<value optimized out>) at misprite.c:902
#3 0x0000000000456d9b in miPointerUpdateSprite (pDev=0x26877c0) at mipointer.c:402
#4 0x0000000000456ed3 in miPointerSetCursorPosition (pDev=0x26877c0, pScreen=0x2388d50, x=0, y=0, generateEvent=0) at mipointer.c:241
#5 0x00000000004b06ef in AnimCurSetCursorPosition (pDev=0x26877c0, pScreen=0x2388d50, x=0, y=0, generateEvent=<value optimized out>)
#6 0x00000000004586bf in mieqProcessDeviceEvent (dev=0x26877c0, event=0x2844c20, screen=0x2388d50) at mieq.c:388
#7 0x000000000045876a in mieqProcessInputEvents () at mieq.c:471
#8 0x0000000000467a38 in ProcessInputEvents () at xf86Events.c:165
#9 0x000000000044d99a in Dispatch () at dispatch.c:371
#10 0x0000000000424f4a in main (argc=3, argv=0x7bc3a0, envp=<value optimized out>) at main.c:285
If there's information missing (the optimised out stuff) let me know. I can recompile without optimisation.
For reference that was done with -ggdb in CFLAGS and splitdebug in features.
Please attach your Xorg.0.log from after the crash.
I can no longer reproduce this since I've changed hardware (different GPU)
I now have X just not start if I turn Xinerama.
Should I attach the Xorg.0.log or should I start a new bug?
Yes, please file a new bug. Don't forget to attach your Xorg.0.log.