Addurl can fail due to an apparent race condition when watch or assistant is running and the repository is in direct mode. The following stress test script encounters the bug consistently on my system. I am running git-annex 5.20140320 on on Ubuntu 13.10.

[[!format sh """
#!/bin/sh
set -eu

cleanup() {
  local dir
  dir="$1"; shift
  if [ -d "$dir" ]; then
    (
      set -x
      fuser -k -w "$dir/annex/.git/annex/daemon.log" || :
      find "$dir" -type d -exec chmod 700 '{}' '+'
      find "$dir" -type f -exec chmod 600 '{}' '+'
      rm -fr "$dir"
    )
  fi
}

go() {
  local dir
  dir="$(mktemp -d "${TMP:-/tmp}/stress-annex.XXXXXXXXXX")"
  trap "cleanup '$dir'" 0 1 2 13 15

  (
    cd "$dir"
    mkdir annex
    cd annex
    set -x

    git init
    git annex init
    git annex direct
    git annex watch

    for n in $(seq 100); do
      git annex addurl --file=foo http://heh.fi/robots.txt
      git annex sync
      rm -f foo
      git annex sync
    done

    git annex watch --stop
    git annex uninit
  )

  cleanup "$dir"
  trap - 0 1 2 13 14
}

go
"""]]

Script output:

[[!format sh """
% ./stress-annex
+ git init
Initialized empty Git repository in /tmp/stress-annex.OKj6D8kVmV/annex/.git/
+ git annex init
init  ok
(Recording state in git...)
+ git annex direct
commit  
On branch master

Initial commit

nothing to commit
ok
direct  ok
+ git annex watch
+ seq 100
+ git annex addurl --file=foo http://heh.fi/robots.txt
addurl foo (downloading http://heh.fi/robots.txt ...) 
--2014-03-27 03:14:29--  http://heh.fi/robots.txt
Resolving heh.fi (heh.fi)... 83.145.237.222
Connecting to heh.fi (heh.fi)|83.145.237.222|:80... connected.
HTTP request sent, awaiting response... 200 OK
Length: 0 [text/plain]
Saving to: ‘/tmp/stress-annex.OKj6D8kVmV/annex/.git/annex/tmp/URL--http&c%%heh.fi%robots.txt’

    [ <=>                                                                                      ] 0           --.-K/s   in 0s      

2014-03-27 03:14:29 (0.00 B/s) - ‘/tmp/stress-annex.OKj6D8kVmV/annex/.git/annex/tmp/URL--http&c%%heh.fi%robots.txt’ saved [0/0]

(Recording state in git...)
ok
(Recording state in git...)
+ git annex sync
commit  ok
+ rm -f foo
+ git annex sync
commit  (Recording state in git...)
ok
(Recording state in git...)
+ git annex addurl --file=foo http://heh.fi/robots.txt
addurl foo (downloading http://heh.fi/robots.txt ...) 
--2014-03-27 03:14:29--  http://heh.fi/robots.txt
Resolving heh.fi (heh.fi)... 83.145.237.222
Connecting to heh.fi (heh.fi)|83.145.237.222|:80... connected.
HTTP request sent, awaiting response... 200 OK
Length: 0 [text/plain]
Saving to: ‘/tmp/stress-annex.OKj6D8kVmV/annex/.git/annex/tmp/URL--http&c%%heh.fi%robots.txt’

    [ <=>                                                                                      ] 0           --.-K/s   in 0s      

2014-03-27 03:14:29 (0.00 B/s) - ‘/tmp/stress-annex.OKj6D8kVmV/annex/.git/annex/tmp/URL--http&c%%heh.fi%robots.txt’ saved [0/0]

(Recording state in git...)
ok
(Recording state in git...)
+ git annex sync
commit  ok
+ rm -f foo
+ git annex sync
commit  (Recording state in git...)
ok
(Recording state in git...)
+ git annex addurl --file=foo http://heh.fi/robots.txt
addurl foo (downloading http://heh.fi/robots.txt ...) 
--2014-03-27 03:14:29--  http://heh.fi/robots.txt
Resolving heh.fi (heh.fi)... 83.145.237.222
Connecting to heh.fi (heh.fi)|83.145.237.222|:80... connected.
HTTP request sent, awaiting response... 200 OK
Length: 0 [text/plain]
Saving to: ‘/tmp/stress-annex.OKj6D8kVmV/annex/.git/annex/tmp/URL--http&c%%heh.fi%robots.txt’

    [ <=>                                                                                      ] 0           --.-K/s   in 0s      

2014-03-27 03:14:29 (0.00 B/s) - ‘/tmp/stress-annex.OKj6D8kVmV/annex/.git/annex/tmp/URL--http&c%%heh.fi%robots.txt’ saved [0/0]


git-annex: /tmp/stress-annex.OKj6D8kVmV/annex/.git/annex/objects/pX/ZJ/SHA256E-s0--e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855/: openTempFile: permission denied (Permission denied)
failed
git-annex: addurl: 1 failed
+ fuser -k -w /tmp/stress-annex.OKj6D8kVmV/annex/.git/annex/daemon.log
/tmp/stress-annex.OKj6D8kVmV/annex/.git/annex/daemon.log: 30704 30709 30735 30738 30778
+ find /tmp/stress-annex.OKj6D8kVmV -type d -exec chmod 700 {} +
+ find /tmp/stress-annex.OKj6D8kVmV -type f -exec chmod 600 {} +
+ rm -fr /tmp/stress-annex.OKj6D8kVmV
"""]]

The script also seems to encounter another issue. The output when seq 100 is changed to seq 1 and addurl happens to succeed:

[[!format sh """
+ git init
Initialized empty Git repository in /tmp/stress-annex.QEs0pNyS9z/annex/.git/
+ git annex init
init  ok
(Recording state in git...)
+ git annex direct
commit  
On branch master

Initial commit

nothing to commit
ok
direct  ok
+ git annex watch
+ seq 1
+ git annex addurl --file=foo http://heh.fi/robots.txt
addurl foo (downloading http://heh.fi/robots.txt ...) 
--2014-03-27 03:17:20--  http://heh.fi/robots.txt
Resolving heh.fi (heh.fi)... 83.145.237.222
Connecting to heh.fi (heh.fi)|83.145.237.222|:80... connected.
HTTP request sent, awaiting response... 200 OK
Length: 0 [text/plain]
Saving to: ‘/tmp/stress-annex.QEs0pNyS9z/annex/.git/annex/tmp/URL--http&c%%heh.fi%robots.txt’

    [ <=>                                                                                      ] 0           --.-K/s   in 0s      

2014-03-27 03:17:20 (0.00 B/s) - ‘/tmp/stress-annex.QEs0pNyS9z/annex/.git/annex/tmp/URL--http&c%%heh.fi%robots.txt’ saved [0/0]

(Recording state in git...)
ok
(Recording state in git...)
+ git annex sync
commit  ok
+ rm -f foo
+ git annex sync
commit  (Recording state in git...)
ok
(Recording state in git...)
+ git annex watch --stop
+ git annex uninit
git-annex: /tmp/stress-annex.QEs0pNyS9z/annex/.git/annex/objects/pX/ZJ/SHA256E-s0--e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855/SHA256E-s0--e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855.map: removeLink: permission denied (Permission denied)
+ fuser -k -w /tmp/stress-annex.QEs0pNyS9z/annex/.git/annex/daemon.log
+ :
+ find /tmp/stress-annex.QEs0pNyS9z -type d -exec chmod 700 {} +
+ find /tmp/stress-annex.QEs0pNyS9z -type f -exec chmod 600 {} +
+ rm -fr /tmp/stress-annex.QEs0pNyS9z
"""]]