diff --git a/hgext/blackbox.py b/hgext/blackbox.py
new file mode 100644
--- /dev/null
+++ b/hgext/blackbox.py
@@ -0,0 +1,38 @@
+"""log commands ran in the repository and exceptions encountered
+
+Appends to .hg/blackbox events happening in the repository to help
+debugging and diagnosis. Each command that is run in the repository,
+or each exception encountered will be logged.
+"""
+
+from mercurial.i18n import _
+from mercurial import extensions, dispatch
+import traceback
+
+def runcommand(orig, lui, repo, cmd, fullargs, ui, options, d, cmdpats, cmdoptions):
+    if repo is None:
+        # not much we can log
+        return orig(lui, repo, cmd, fullargs, ui, options, d, cmdpats, cmdoptions)
+    blackboxfile = repo.opener('blackbox', 'a')
+    try:
+        blackboxfile.write('%s\n' % ' '.join(fullargs))
+        try:
+            return orig(lui, repo, cmd, fullargs, ui, options, d, cmdpats, cmdoptions)
+        except:
+            traceback.print_exc(None, blackboxfile)
+            raise
+    finally:
+        blackboxfile.close()
+
+def incoming(ui, repo, hooktype, node, source, url):
+    blackboxfile = repo.opener('blackbox', 'a')
+    try:
+        blackboxfile.write('incoming: %s: %s from %s\n' % (source, node[:12], url))
+    finally:
+        blackboxfile.close()
+
+def uisetup(ui):
+    extensions.wrapfunction(dispatch, 'runcommand', runcommand)
+def reposetup(ui, repo):
+    ui.setconfig('hooks', 'incoming.blackbox', incoming)
+
diff --git a/tests/test-blackbox.t b/tests/test-blackbox.t
new file mode 100644
--- /dev/null
+++ b/tests/test-blackbox.t
@@ -0,0 +1,60 @@
+  $ echo '[extensions]' >> $HGRCPATH
+  $ echo 'blackbox =' >> $HGRCPATH
+
+Create a repo with a dummy changeset
+
+  $ hg init foo; cd foo
+  $ touch a
+  $ hg ci -Am'a'
+  adding a
+  $ hg clone . ../bar
+  updating to branch default
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+
+And pretend we push to remote
+
+  $ echo foo > a
+  $ hg ci -Am'modify a'
+  $ hg push ../bar
+  pushing to ../bar
+  searching for changes
+  adding changesets
+  adding manifests
+  adding file changes
+  added 1 changesets with 1 changes to 1 files
+
+Log a few commands
+
+  $ cd ../bar
+  $ hg foo > /dev/null
+  hg: unknown command 'foo'
+  [255]
+  $ hg st
+  $ hg diff -c notexisting
+  abort: unknown revision 'notexisting'!
+  [255]
+
+And check that both the incoming hook, exceptions and commands are logged:
+
+  $ cat .hg/blackbox
+  incoming: push: c3cdbe7bbdc4 from file:$TESTTMP/foo
+  st
+  diff -c notexisting
+  Traceback (most recent call last):
+    File "*/hgext/blackbox.py", line *, in runcommand (glob)
+      return orig(lui, repo, cmd, fullargs, ui, options, d, cmdpats, cmdoptions)
+    File "*/mercurial/dispatch.py", line *, in runcommand (glob)
+      ret = _runcommand(ui, options, cmd, d)
+    File "*/mercurial/dispatch.py", line *, in _runcommand (glob)
+      return checkargs()
+    File "*/mercurial/dispatch.py", line *, in checkargs (glob)
+      return cmdfunc()
+    File "*/mercurial/dispatch.py", line *, in <lambda> (glob)
+      d = lambda: util.checksignature(func)(ui, *args, **cmdoptions)
+    File "*/mercurial/util.py", line *, in check (glob)
+      return func(*args, **kwargs)
+    File "*/mercurial/commands.py", line *, in diff (glob)
+      node2 = repo.lookup(change)
+    File "*/mercurial/localrepo.py", line *, in lookup (glob)
+      raise error.RepoLookupError(_("unknown revision '%s'") % key)
+  RepoLookupError: unknown revision 'notexisting'