I can reliably get a crash without changing coding systems:

(progn
  (buffer-swap-text (generate-new-buffer "test"))
  (insert (make-string 128 ?a)))

(progn
  (buffer-swap-text (generate-new-buffer "test"))
  (garbage-collect))

All the crashes I've seen with tar-mode or
buffer-swap-text end up hitting the same call to abort in
r_re_alloc().

Backtraces for the above lisp fragments (same order):

(gdb) bt
#0  w32_abort () at w32fns.c:7279
#1  0x01137df8 in r_re_alloc (ptr=0x2a27008, size=2129) at ralloc.c:1028
#2  0x0106ed4e in enlarge_buffer_text (b=0x2a27000, delta=2108) at
buffer.c:5065
#3  0x010fe25e in make_gap_larger (nbytes_added=2108) at insdel.c:526
#4  0x010ff949 in insert_from_string_1 (string=47277539, pos=0, pos_byte=0,
    nchars=128, nbytes=128, inherit=0, before_markers=44199944) at
insdel.c:1107
#5  0x01100da6 in insert_from_string (string=47277539, pos=0, pos_byte=0,
length=128,
    length_byte=0, inherit=44199944) at insdel.c:1048
#6  0x0108d79c in general_insert_function (insert_func=0x1100fac <insert>,
    insert_from_string_func=0x1100d3b <insert_from_string>, inherit=0,
nargs=1,
    args=0x82f3e0) at editfns.c:2184
#7  0x0108d86c in Finsert (nargs=0, args=0x0) at editfns.c:2228
#8  0x0100b364 in Feval (form=19643352) at eval.c:2378
#9  0x0100b632 in Fprogn (args=0) at eval.c:449
#10 0x0100b42b in Feval (form=18325836) at eval.c:2322
#11 0x0100be38 in Ffuncall (nargs=2, args=0x1179ef0) at eval.c:3044
#12 0x01111e63 in Fbyte_code (bytestr=0, vector=8582708, maxdepth=1) at
bytecode.c:678
#13 0x0100b722 in funcall_lambda (fun=18972820, nargs=1,
arg_vector=0x82f774)
    at eval.c:3231
#14 0x0100bc17 in Ffuncall (nargs=2, args=0x1218094) at eval.c:3101
#15 0x01111e63 in Fbyte_code (bytestr=0, vector=8583024, maxdepth=1) at
bytecode.c:678
#16 0x0100b722 in funcall_lambda (fun=18973068, nargs=1,
arg_vector=0x82f8b4)
    at eval.c:3231
#17 0x0100bc17 in Ffuncall (nargs=2, args=0x121818c) at eval.c:3101
#18 0x01111e63 in Fbyte_code (bytestr=0, vector=8583344, maxdepth=1) at
bytecode.c:678
#19 0x0100b722 in funcall_lambda (fun=18971276, nargs=0,
arg_vector=0x82fa24)
    at eval.c:3231
#20 0x0100bc17 in Ffuncall (nargs=1, args=0x1217a8c) at eval.c:3101
#21 0x0100d37f in apply1 (fn=53934297, arg=0) at eval.c:2785
#22 0x0110fdc5 in Fcall_interactively (function=53934297,
record_flag=44161025,
    keys=44194564) at callint.c:389
#23 0x0100be11 in Ffuncall (nargs=4, args=0x12bf728) at eval.c:3050
#24 0x0100bfe9 in call3 (fn=0, arg1=0, arg2=0, arg3=0) at eval.c:2870
#25 0x01056cb9 in Fcommand_execute (cmd=53934297, record_flag=44161025,
keys=0,
    special=44161025) at keyboard.c:10333
#26 0x0105e4e5 in command_loop_1 () at keyboard.c:1880
#27 0x01009f9e in internal_condition_case (bfun=0x105e180 <command_loop_1>,
    handlers=44224777, hfun=0x105772c <cmd_error>) at eval.c:1511
#28 0x01051cba in command_loop_2 () at keyboard.c:1338
#29 0x01009ed3 in internal_catch (tag=0, func=0x1051c97 <command_loop_2>,
arg=44161025)
    at eval.c:1247
#30 0x01051ac7 in command_loop () at keyboard.c:1317
#31 0x01051b60 in recursive_edit_1 () at keyboard.c:942
#32 0x01051c81 in Frecursive_edit () at keyboard.c:1004
#33 0x01002e36 in main (argc=1, argv=0xa427b8) at emacs.c:1777

Lisp Backtrace:
"insert" (0x82f3e0)
"progn" (0x82f518)
"eval" (0x82f638)
"eval-last-sexp-1" (0x82f774)
"eval-last-sexp" (0x82f8b4)
"eval-print-last-sexp" (0x82fa24)
"call-interactively" (0x82fc04)
(gdb)


(gdb) bt
#0  w32_abort () at w32fns.c:7279
#1  0x01137df8 in r_re_alloc (ptr=0x2d56008, size=100) at ralloc.c:1028
#2  0x0106ed4e in enlarge_buffer_text (b=0x2d56000, delta=-1979) at
buffer.c:5065
#3  0x010fe38a in make_gap_smaller (nbytes_removed=-1979) at insdel.c:600
#4  0x01065cab in Fgarbage_collect () at alloc.c:5022
#5  0x0100b383 in Feval (form=18331376) at eval.c:2372
#6  0x0100b632 in Fprogn (args=0) at eval.c:449
#7  0x0100b42b in Feval (form=18325836) at eval.c:2322
#8  0x0100be38 in Ffuncall (nargs=2, args=0x1179ef0) at eval.c:3044
#9  0x01111e63 in Fbyte_code (bytestr=0, vector=8582708, maxdepth=1) at
bytecode.c:678
#10 0x0100b722 in funcall_lambda (fun=18972820, nargs=1,
arg_vector=0x82f774)
    at eval.c:3231
#11 0x0100bc17 in Ffuncall (nargs=2, args=0x1218094) at eval.c:3101
#12 0x01111e63 in Fbyte_code (bytestr=0, vector=8583024, maxdepth=1) at
bytecode.c:678
#13 0x0100b722 in funcall_lambda (fun=18973068, nargs=1,
arg_vector=0x82f8b4)
    at eval.c:3231
#14 0x0100bc17 in Ffuncall (nargs=2, args=0x121818c) at eval.c:3101
#15 0x01111e63 in Fbyte_code (bytestr=0, vector=8583344, maxdepth=1) at
bytecode.c:678
#16 0x0100b722 in funcall_lambda (fun=18971276, nargs=0,
arg_vector=0x82fa24)
    at eval.c:3231
#17 0x0100bc17 in Ffuncall (nargs=1, args=0x1217a8c) at eval.c:3101
#18 0x0100d37f in apply1 (fn=53934297, arg=0) at eval.c:2785
#19 0x0110fdc5 in Fcall_interactively (function=53934297,
record_flag=44161025,
    keys=44194564) at callint.c:389
#20 0x0100be11 in Ffuncall (nargs=4, args=0x12bf728) at eval.c:3050
#21 0x0100bfe9 in call3 (fn=0, arg1=0, arg2=0, arg3=0) at eval.c:2870
#22 0x01056cb9 in Fcommand_execute (cmd=53934297, record_flag=44161025,
keys=0,
    special=44161025) at keyboard.c:10333
#23 0x0105e4e5 in command_loop_1 () at keyboard.c:1880
#24 0x01009f9e in internal_condition_case (bfun=0x105e180 <command_loop_1>,
    handlers=44224777, hfun=0x105772c <cmd_error>) at eval.c:1511
#25 0x01051cba in command_loop_2 () at keyboard.c:1338
#26 0x01009ed3 in internal_catch (tag=0, func=0x1051c97 <command_loop_2>,
arg=44161025)
    at eval.c:1247
#27 0x01051ac7 in command_loop () at keyboard.c:1317
#28 0x01051b60 in recursive_edit_1 () at keyboard.c:942
#29 0x01051c81 in Frecursive_edit () at keyboard.c:1004
#30 0x01002e36 in main (argc=1, argv=0xa427b8) at emacs.c:1777

Lisp Backtrace:
"garbage-collect" (0x82f420)
"progn" (0x82f518)
"eval" (0x82f638)
"eval-last-sexp-1" (0x82f774)
"eval-last-sexp" (0x82f8b4)
"eval-print-last-sexp" (0x82fa24)
"call-interactively" (0x82fc04)
(gdb)