This backtrace comes up reliably if I run ab -c 10 against a hunchentoot instance I've running on ccl 1.3-dev-r11912  (DarwinX8664). ab -c 1 works perfectly.

value #<STRING-OUTPUT-STREAM  #x300041E811ED> is not of the expected type STRUCTURE.
   [Condition of type TYPE-ERROR]

Restarts:
 0: [ABORT-BREAK] Reset this thread
 1: [ABORT] Kill this thread

Backtrace:
  0: (#<Anonymous Function #x30004100FF3F> #<Compiled-function SWANK:SWANK-DEBUGGER-HOOK #x3000410C047F> #<COMPILED-LEXICAL-CLOSURE (:INTERNAL SWANK:INVOKE-SLIME-DEBUGGER) #x300041FB433F>)
  1: (SWANK::CALL-WITH-BINDINGS ((*STANDARD-OUTPUT* . #<SWANK-BACKEND::SLIME-OUTPUT-STREAM #x30004111958D>) (*STANDARD-INPUT* . #<SWANK-BACKEND::SLIME-INPUT-STREAM #x30004111154D>) ..)))
  2: (SWANK::CALL-WITH-CONNECTION #<CONNECTION #x3000410F279D> #<COMPILED-LEXICAL-CLOSURE (:INTERNAL SWANK:INVOKE-SLIME-DEBUGGER) #x300041ECCA1F>)
  3: (SWANK:INVOKE-SLIME-DEBUGGER #<TYPE-ERROR #x300041ECCA9D>)
  4: (#<Anonymous Function #x30004100FF3F> #<Compiled-function SWANK:SWANK-DEBUGGER-HOOK #x3000410C047F> #<COMPILED-LEXICAL-CLOSURE (:INTERNAL SWANK:SWANK-DEBUGGER-HOOK) #x300041ECCA5F>)
  5: (SWANK:SWANK-DEBUGGER-HOOK #<TYPE-ERROR #x300041ECCA9D> #<Compiled-function SWANK:SWANK-DEBUGGER-HOOK #x3000410C047F>)
  6: (CCL::BREAK-LOOP-HANDLE-ERROR #<TYPE-ERROR #x300041ECCA9D> 3190155)
  7: (CCL::%ERROR #<TYPE-ERROR #x300041ECCA9D> NIL 3190155)
  8: ((:INTERNAL CCL::%XERR-DISP))
  9: (CCL::FUNCALL-WITH-ERROR-REENTRY-DETECTION #<COMPILED-LEXICAL-CLOSURE (:INTERNAL CCL::%XERR-DISP) #x196787F>)
 10: (CCL::%XERR-DISP 370385030)
 11: (CCL::%PASCAL-FUNCTIONS% 2 370385030)
 12: (CCL::CREATE-STRING-OUTPUT-STREAM-IOBLOCK :STREAM #<STRING-OUTPUT-STREAM  :CLOSED #x300041ECCB2D> :STRING "" :ELEMENT-TYPE CHARACTER :WRITE-CHAR-FUNCTION CCL::STRING-OUTPUT-STREAM-IOBLOCK-WRITE-CHAR :..
 13: (CCL::%%MAKE-STRING-OUTPUT-STREAM #<BUILT-IN-CLASS STRING-OUTPUT-STREAM> "" CCL::STRING-OUTPUT-STREAM-IOBLOCK-WRITE-CHAR CCL::STRING-OUTPUT-STREAM-IOBLOCK-WRITE-SIMPLE-STRING)
 14: (FORMAT #<STRING-OUTPUT-STREAM  :CLOSED #x300041ECCB2D> "~A.~A.~A.~A" 127 0 0 1)
 15: (HUNCHENTOOT::CLIENT-AS-STRING #<USOCKET:STREAM-USOCKET #x300041ECCBDD>)
      Locals:
        HUNCHENTOOT::SOCKET = #<USOCKET:STREAM-USOCKET #x300041ECCBDD>
        HUNCHENTOOT::ADDRESS = #(127 0 0 1)
        HUNCHENTOOT::PORT = 53102
 16: (#<STANDARD-METHOD HUNCHENTOOT:HANDLE-INCOMING-CONNECTION (HUNCHENTOOT:ONE-THREAD-PER-CONNECTION-TASKMASTER ..)
 17: (#<STANDARD-METHOD HUNCHENTOOT:ACCEPT-CONNECTIONS (HUNCHENTOOT:ACCEPTOR)> #<DEBUGGABLE-ACCEPTOR (host *, port 4242)>)
 18: ((:INTERNAL BORDEAUX-THREADS::BINDING-DEFAULT-SPECIALS))
 19: (CCL::RUN-PROCESS-INITIAL-FORM #<PROCESS Hunchentoot listener (*:4242)(57) [Reset] #x300041ECA7ED> (#<COMPILED-LEXICAL-CLOSURE (:INTERNAL CCL::%PROCESS-RUN-FUNCTION) #x300041ECA58F>))
 20: ((:INTERNAL ..))
 21: ((:INTERNAL CCL::THREAD-MAKE-STARTUP-FUNCTION))

(cl:in-package :cl-user)

(hunchentoot:start (make-instance 'hunchentoot:acceptor :port 4242))

(hunchentoot:define-easy-handler (static-json :uri "/bench/json/dummy")
    ()
  "[[[1114370936000, 3.016666666666667e-1], [1117131100000, 3.12e-1], [1119903045000, 3.1266666666666665e-1], [1122330766000, 3.1266666666666665e-1], [1125075618000, 3.12e-1], [1127825895000, 3.116666666666667e-1], [1130373796000, 3.113333333333333e-1], [1133196772000, 3.113333333333333e-1], [1135700334000, 3.08e-1], [1138300151000, 3.12e-1], [1141006083000, 3.1066666666666665e-1], [1143413954000, 3.013333333333333e-1], [1146065363000, 3.013333333333333e-1], [1148759200000, 3.036666666666667e-1], [1151343632000, 3.0e-1], [1153943420000, 3.0e-1], [1156537602000, 3.0266666666666664e-1], [1159288015000, 2.9866666666666664e-1], [1161784720000, 2.9866666666666664e-1], [1164850603000, 3.0e-1], [1167141464000, 3.1066666666666665e-1], [1169736694000, 3.053333333333333e-1], [1172603022000, 3.16e-1], [1174837194000, 3.053333333333333e-1], [1177782478000, 3.1866666666666665e-1], [1180224808000, 3.16e-1], [1182984249000, 3.133333333333333e-1], [1185367173000, 3.093333333333333e-1], [1188169815000, 3.093333333333333e-1], [1190733504000, 3.053333333333333e-1], [1193344083000, 3.1066666666666665e-1], [1196033635000, 3.1466666666666665e-1], [1198757549000, 3.0266666666666664e-1], [1201469587000, 3.133333333333333e-1], [1204033707000, 3.08e-1], [1208047141000, 3.12e-1], [1211972404000, 3.1066666666666665e-1], [1214766685000, 3.133333333333333e-1], [1220231141000, 3.12e-1], [1222951242000, 2.9466666666666663e-1], [1225400326000, 3.013333333333333e-1], [1228140273000, 3.0e-1], [1230726441000, 3.0266666666666664e-1], [1233625752000, 2.92e-1]], [[1114370936000, 3.0217584174388123e-1, 3.0115749158945215e-1], [1117131100000, 3.1266666666666854e-1, 3.1133333333333146e-1], [1119903045000, 3.1336055533315643e-1, 3.1197277800017686e-1], [1122330766000, 3.130515668461293e-1, 3.12281766487204e-1], [1125075618000, 3.123333333333381e-1, 3.116666666666619e-1], [1127825895000, 3.1236055533315504e-1, 3.1097277800017836e-1], [1130373796000, 3.1171823351279326e-1, 3.109484331538733e-1], [1133196772000, 3.121722038261422e-1, 3.104944628405244e-1], [1135700334000, 3.09333333333334e-1, 3.06666666666666e-1], [1138300151000, 3.13333333333334e-1, 3.10666666666666e-1], [1141006083000, 3.114364670255865e-1, 3.0989686630774677e-1], [1143413954000, 3.021031336922545e-1, 3.005635329744121e-1], [1146065363000, 3.018425084105519e-1, 3.008241582561147e-1], [1148759200000, 3.0468501682110083e-1, 3.0264831651223256e-1], [1151343632000, 3.0133333333333323e-1, 2.9866666666666675e-1], [1153943420000, 3.023094010767592e-1, 2.976905989232408e-1], [1156537602000, 3.034364670255865e-1, 3.0189686630774676e-1], [1159288015000, 2.994364670255865e-1, 2.9789686630774675e-1], [1161784720000, 3.0174586810234516e-1, 2.955874652309881e-1], [1164850603000, 3.0133333333333323e-1, 2.9866666666666675e-1], [1167141464000, 3.114364670255865e-1, 3.0989686630774677e-1], [1169736694000, 3.061031336922545e-1, 3.045635329744121e-1], [1172603022000, 3.1952766841475283e-1, 3.124723315852472e-1], [1174837194000, 3.068729340511724e-1, 3.037937326154942e-1], [1177782478000, 3.194364670255852e-1, 3.178968663077481e-1], [1180224808000, 3.1952766841475283e-1, 3.124723315852472e-1], [1182984249000, 3.1537003364220256e-1, 3.1129663302446403e-1], [1185367173000, 3.101031336922545e-1, 3.085635329744121e-1], [1188169815000, 3.108729340511724e-1, 3.077937326154942e-1], [1190733504000, 3.0737003364220306e-1, 3.032966330244635e-1], [1193344083000, 3.114364670255865e-1, 3.0989686630774677e-1], [1196033635000, 3.1543646702558653e-1, 3.1389686630774677e-1], [1198757549000, 3.047033669755359e-1, 3.006299663577974e-1], [1201469587000, 3.1537003364220256e-1, 3.1129663302446403e-1], [1204033707000, 3.080000020277912e-1, 3.079999979722088e-1], [1208047141000, 3.120000014338649e-1, 3.119999985661351e-1], [1211972404000, 3.127033669755364e-1, 3.086299663577969e-1], [1214766685000, 3.1487293405117306e-1, 3.1179373261549354e-1], [1220231141000, 3.120000014338649e-1, 3.119999985661351e-1], [1222951242000, 2.967033669755364e-1, 2.9262996635779687e-1], [1225400326000, 3.021031336922545e-1, 3.005635329744121e-1], [1228140273000, 3.01333333333334e-1, 2.9866666666666597e-1], [1230726441000, 3.042062673845064e-1, 3.011270659488269e-1], [1233625752000, 2.943094010767592e-1, 2.896905989232408e-1]]]")

;;; Now run:
;;;; ab -c 100 -n 2000 http://localhost:4242/bench/json/dummy
;;;; should result in hilarity.

This is ApacheBench, Version 2.0.40-dev <$Revision: 1.146 $> apache-2.0
Copyright 1996 Adam Twiss, Zeus Technology Ltd, http://www.zeustech.net/
Copyright 2006 The Apache Software Foundation, http://www.apache.org/

Benchmarking localhost (be patient)
Completed 200 requests
Completed 400 requests
Completed 600 requests
Completed 800 requests
Completed 1000 requests
Completed 1200 requests
Completed 1400 requests
Completed 1600 requests
Completed 1800 requests
Finished 2000 requests


Server Software:        Hunchentoot
Server Hostname:        localhost
Server Port:            4242

Document Path:          /bench/json/dummy
Document Length:        4260 bytes

Concurrency Level:      100
Time taken for tests:   2.984606 seconds
Complete requests:      2000
Failed requests:        0
Write errors:           0
Total transferred:      8858000 bytes
HTML transferred:       8520000 bytes
Requests per second:    670.11 [#/sec] (mean)
Time per request:       149.230 [ms] (mean)
Time per request:       1.492 [ms] (mean, across all concurrent requests)
Transfer rate:          2898.20 [Kbytes/sec] received

Connection Times (ms)
              min  mean[+/-sd] median   max
Connect:        0    0   0.1      0       2
Processing:     5   11  38.3      8     936
Waiting:        4    9  23.9      7     898
Total:          5   11  38.3      8     936

Percentage of the requests served within a certain time (ms)
  50%      8
  66%      9
  75%      9
  80%      9
  90%     11
  95%     19
  98%     61
  99%     66
 100%    936 (longest request)