found something that crashes consistently: the xscr hacks that use threads.
this from the 9vx console:
783 substrate fault 0x9a1 no segment
segment 0xf000000 0x10000000
segment 0x1000 0x1e000
segment 0x1e000 0x27000
segment 0x27000 0x251000
783 substrate: unhandled fault va=9a1 [10009a1] eip=13f07
cpu0: registers for substrate 783
FLAGS=0 TRAP=0 ECODE=0 PC=13F07 USP=3730C
AX 0000030F BX 00000031 CX 00026B08 DX 0000030F
SI 0000030F DI 00037308 BP 00000000
785 strange: unhandled fault va=37424000 [38424000] eip=15014
cpu0: registers for strange 785
FLAGS=0 TRAP=0 ECODE=0 PC=15014 USP=28594
AX 00000001 BX 83C33CC4 CX 0002061C DX 00000311
SI 00000311 DI 00028590 BP 00000000
inside 9vx:
% xscr/substrate
substrate 783: sxscruicide: sys: trap: page fault pc=0x00013f07
% xscr/strange
strange 785: suicide: sys: trap: page fault pc=0x00015014
% acid 785
/proc/785/text:386 plan 9 executable
/sys/lib/acid/port
/sys/lib/acid/386
acid: lstk()
threadexitsall(exitstr=0x1ee21)+0xc9 /sys/src/libthread/exit.c:49
mypid=0x311
npid=0x0
pid=0x1c4ac
i=0x91b1
threadmain(argv=0xfffffb0,argc=0x0)+0xee /usr/andrey/xscr/strange.c:404
_argc=0xfefefefe
_args=0xfefefefe
m=0x0
mainlauncher(arg=0x25cc0)+0x18 /sys/src/libthread/main.c:58
launcher386(arg=0x25cc0,f=0x1495c)+0x10 /sys/src/libthread/386.c:10
0xfefefefe ?file?:0
acid:
echo kill > /proc/785/ctl
% acid 783
/proc/783/text:386 plan 9 executable
/sys/lib/acid/port
/sys/lib/acid/386
acid: lstk()
threadexitsall(exitstr=0x1e7cd)+0xc9 /sys/src/libthread/exit.c:49
mypid=0x30f
npid=0x0
pid=0x1e4fc
i=0x81aa
screenhack()+0x15a /usr/andrey/xscr/substrate.c:381
max_cycles=0x7d0
tempx=0x63
threadmain(argv=0xfffffb0,argc=0x0)+0xce /usr/andrey/xscr/substrate.c:448
_argc=0xfefefefe
_args=0xfefefefe
mainlauncher(arg=0x34a60)+0x18 /sys/src/libthread/main.c:58
launcher386(arg=0x34a60,f=0x1384f)+0x10 /sys/src/libthread/386.c:10
0xfefefefe ?file?:0
acid:
% xscr/swirl
swirl 803: suicide: sys: trap: page fault pc=0x0001546a
% xscr/zoom
zoom 805: suicide: sys: trap: page fault pc=0x000108d2
%
