小澤といいます。
以下のスクリプトがSegfaultしました。

while true
  t = Thread.start {
    a
  }
  t.abort_on_exception = true
end

[kuma@opera prog]$ ruby sf.rb
sf.rb:3: undefined local variable or method `a' for #<Object:0x4029aaec> (NameError)
        from sf.rb:2:in `start'
        from sf.rb:2
sf.rb:2: no implicit conversion to integer from nil (TypeError)
sf.rb:2: [BUG] Segmentation fault
ruby 1.7.2 (2002-04-04) [i686-linux]
アボートしました (core dumped)

[kuma@opera prog]$ gdb ruby
GNU gdb 20010318
Copyright 2001 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB.  Type "show warranty" for details.
This GDB was configured as "i586-redhat-linux"...
(gdb) set args sf.rb
(gdb) r
Starting program: /home/kuma/bin/ruby sf.rb
sf.rb:3: undefined local variable or method `a' for #<Object:0x4029aaec> (NameError)
        from sf.rb:2:in `start'
        from sf.rb:2
sf.rb:2: no implicit conversion to integer from nil (TypeError)

Program received signal SIGSEGV, Segmentation fault.
rb_longjmp (tag=6, mesg=1076388576) at ../eval.c:3575
3575        JUMP_TAG(tag);
(gdb) bt
#0  rb_longjmp (tag=6, mesg=1076388576) at ../eval.c:3575
#1  0x4003dff1 in rb_exc_raise () at ../eval.c:3582
#2  0x40034775 in rb_raise () at ../error.c:668
#3  0x4006563b in rb_num2long (val=4) at ../numeric.c:770
#4  0x40036dcf in error_handle (ex=6) at ../eval.c:1132
#5  0x4003704d in ruby_stop (ex=6) at ../eval.c:1201
#6  0x4003716b in ruby_run () at ../eval.c:1225
#7  0x080487df in main (argc=2, argv=0xbffff6cc, envp=0xbffff6d8)
    at ../main.c:50
#8  0x401591be in __libc_start_main (main=0x80487b0 <main>, argc=2, 
    ubp_av=0xbffff6cc, init=0x80485c8 <_init>, fini=0x804884c <_fini>, 
    rtld_fini=0x4000ddf0 <_dl_fini>, stack_end=0xbffff6bc)
    at ../sysdeps/generic/libc-start.c:129
(gdb)