In 4.12.12 on fc26.
X hang and hang again randomly, and logged like this:

nouveau 0000:01:00.0: gr: DATA_ERROR 00000005 [INVALID_ENUM] ch 18 [007f294000
Xorg[2256]] subc 0 class a197 mthd 19c8 data 00000000
nouveau 0000:01:00.0: gr: ILLEGAL_MTHD ch 18 [007f294000 Xorg[2256]] subc 0
class a197 mthd 19d8 data 200308e0
nouveau 0000:01:00.0: gr: ILLEGAL_MTHD ch 18 [007f294000 Xorg[2256]] subc 0
class a197 mthd 19dc data 00000800
nouveau 0000:01:00.0: fifo: write fault at 0000000000 engine 00 [GR] client 0c
[GPC0/RAST] reason 02 [PTE] on channel 18 [007f294000 Xorg[2256]]
nouveau 0000:01:00.0: fifo: channel 18: killed
nouveau 0000:01:00.0: fifo: runlist 0: scheduled for recovery
nouveau 0000:01:00.0: fifo: engine 0: scheduled for recovery
nouveau 0000:01:00.0: gr: DATA_ERROR 0000000c [INVALID_BITFIELD] ch 18
[007f294000 Xorg[2256]] subc 0 class a197 mthd 19e4 data 00340000
nouveau 0000:01:00.0: gr: DATA_ERROR 0000000c [INVALID_BITFIELD] ch 18
[007f294000 Xorg[2256]] subc 0 class a197 mthd 19e8 data a01108e3
nouveau 0000:01:00.0: gr: DATA_ERROR 0000000c [INVALID_BITFIELD] ch 18
[007f294000 Xorg[2256]] subc 0 class a197 mthd 19ec data 00000560

tried 4.12.13 on fc26. 

android emulator randomly results:

nouveau: kernel rejected pushbuf: Invalid argument
nouveau: ch6: krec 0 pushes 0 bufs 13 relocs 0
nouveau: ch6: buf 00000000 00000002 00000004 00000004 00000000
nouveau: ch6: buf 00000001 00000048 00000002 00000002 00000000
nouveau: ch6: buf 00000002 00000007 00000002 00000002 00000000
nouveau: ch6: buf 00000003 00000008 00000002 00000002 00000002
nouveau: ch6: buf 00000004 0000000b 00000002 00000002 00000000
nouveau: ch6: buf 00000005 0000000a 00000002 00000002 00000002
nouveau: ch6: buf 00000006 00000006 00000004 00000000 00000004
nouveau: ch6: buf 00000007 0000004c 00000002 00000000 00000002
nouveau: ch6: buf 00000008 0000004d 00000002 00000000 00000002
nouveau: ch6: buf 00000009 00000057 00000004 00000004 00000000
nouveau: ch6: buf 0000000a 00000095 00000002 00000002 00000000
nouveau: ch6: buf 0000000b 00000065 00000002 00000002 00000000
nouveau: ch6: buf 0000000c 00000043 00000002 00000002 00000000
qemu-system-i386: pushbuf.c:727: nouveau_pushbuf_data: Assertion `kref' failed.
Aborted (core dumped)

now , turn driver to xorg-x11-drv-nvidia , it not crashes.
i don't it whether about to this issue , but it must be nouveau's issue and
output similar error logs.