diff --git a/mercurial/bundle2.py b/mercurial/bundle2.py
--- a/mercurial/bundle2.py
+++ b/mercurial/bundle2.py
@@ -145,7 +145,7 @@ import struct
 import urllib
 import string
 
-import changegroup
+import changegroup, error
 from i18n import _
 
 _pack = struct.pack
@@ -730,7 +730,7 @@ def handlechangegroup(op, inpart):
         h = inpart.read(20)
     assert not h
     if heads != op.repo.heads():
-        raise exchange.PushRaced()
+        raise error.PushRaced()
 
 @parthandler('b2x:output')
 def handleoutput(op, inpart):
diff --git a/mercurial/error.py b/mercurial/error.py
--- a/mercurial/error.py
+++ b/mercurial/error.py
@@ -94,3 +94,7 @@ class SignalInterrupt(KeyboardInterrupt)
 
 class SignatureError(Exception):
     pass
+
+class PushRaced(RuntimeError):
+    """An exception raised during unbundling that indicate a push race"""
+
diff --git a/mercurial/exchange.py b/mercurial/exchange.py
--- a/mercurial/exchange.py
+++ b/mercurial/exchange.py
@@ -8,7 +8,7 @@
 from i18n import _
 from node import hex, nullid
 import errno, urllib
-import util, scmutil, changegroup, base85
+import util, scmutil, changegroup, base85, error
 import discovery, phases, obsolete, bookmarks, bundle2
 
 def readbundle(ui, fh, fname, vfs=None):
@@ -708,9 +708,6 @@ def _getbundleextrapart(bundler, repo, s
     """hook function to let extensions add parts to the requested bundle"""
     pass
 
-class PushRaced(RuntimeError):
-    """An exception raised during unbundling that indicate a push race"""
-
 def check_heads(repo, their_heads, context):
     """check if the heads of a repo have been modified
 
@@ -722,8 +719,8 @@ def check_heads(repo, their_heads, conte
             their_heads == ['hashed', heads_hash]):
         # someone else committed/pushed/unbundled while we
         # were transferring data
-        raise PushRaced('repository changed while %s - '
-                        'please try again' % context)
+        raise error.PushRaced('repository changed while %s - '
+                              'please try again' % context)
 
 def unbundle(repo, cg, heads, source, url):
     """Apply a bundle to a repo.
diff --git a/mercurial/localrepo.py b/mercurial/localrepo.py
--- a/mercurial/localrepo.py
+++ b/mercurial/localrepo.py
@@ -133,7 +133,7 @@ class localpeer(peer.peerrepository):
                 stream = util.chunkbuffer(ret.getchunks())
                 ret = bundle2.unbundle20(self.ui, stream)
             return ret
-        except exchange.PushRaced, exc:
+        except error.PushRaced, exc:
             raise error.ResponseError(_('push failed:'), exc.message)
 
     def lock(self):
diff --git a/mercurial/wireproto.py b/mercurial/wireproto.py
--- a/mercurial/wireproto.py
+++ b/mercurial/wireproto.py
@@ -826,5 +826,5 @@ def unbundle(repo, proto, heads):
         else:
             sys.stderr.write("abort: %s\n" % inst)
             return pushres(0)
-    except exchange.PushRaced, exc:
+    except error.PushRaced, exc:
         return pusherr(str(exc))
diff --git a/tests/test-bundle2.t b/tests/test-bundle2.t
--- a/tests/test-bundle2.t
+++ b/tests/test-bundle2.t
@@ -15,6 +15,7 @@ Create an extension to test bundle2 API
   > from mercurial import scmutil
   > from mercurial import discovery
   > from mercurial import changegroup
+  > from mercurial import error
   > cmdtable = {}
   > command = cmdutil.command(cmdtable)
   > 
@@ -59,6 +60,7 @@ Create an extension to test bundle2 API
   >           ('', 'unknown', False, 'include an unknown mandatory part in the bundle'),
   >           ('', 'parts', False, 'include some arbitrary parts to the bundle'),
   >           ('', 'reply', False, 'produce a reply bundle'),
+  >           ('', 'pushrace', False, 'includes a check:head part with unknown nodes'),
   >           ('r', 'rev', [], 'includes those changeset in the bundle'),],
   >          '[OUTPUTFILE]')
   > def cmdbundle2(ui, repo, path=None, **opts):
@@ -75,6 +77,10 @@ Create an extension to test bundle2 API
   >         capsstring = 'ping-pong\nelephants=babar,celeste\ncity%3D%21=celeste%2Cville'
   >         bundler.addpart(bundle2.bundlepart('b2x:replycaps', data=capsstring))
   > 
+  >     if opts['pushrace']:
+  >         dummynode = '01234567890123456789'
+  >         bundler.addpart(bundle2.bundlepart('b2x:check:heads', data=dummynode))
+  > 
   >     revs = opts['rev']
   >     if 'rev' in opts:
   >         revs = scmutil.revrange(repo, opts['rev'])
@@ -132,6 +138,8 @@ Create an extension to test bundle2 API
   >             tr.close()
   >         except KeyError, exc:
   >             raise util.Abort('missing support for %s' % exc)
+  >         except error.PushRaced, exc:
+  >             raise util.Abort('push race')
   >     finally:
   >         if tr is not None:
   >             tr.release()
@@ -601,6 +609,15 @@ Unbundle the reply to get the output:
   remote: replying to ping request (id 6)
   0 unread bytes
 
+Test push race detection
+
+  $ hg bundle2 --pushrace ../part-race.hg2
+
+  $ hg unbundle2 < ../part-race.hg2
+  0 unread bytes
+  abort: push race
+  [255]
+
 Support for changegroup
 ===================================