--- a/hgext/rebase.py	Sat Jul 16 15:24:28 2011 +0300
+++ b/hgext/rebase.py	Mon Jul 18 22:58:21 2011 +0200
@@ -175,6 +175,14 @@
             assert not extrafn, 'cannot use both keepbranches and extrafn'
             def extrafn(ctx, extra):
                 extra['branch'] = ctx.branch()
+            if collapsef:
+                branches = set()
+                for rev in state:
+                    branches.add(repo[rev].branch())
+                    if len(branches) > 1:
+                        raise util.Abort(_('cannot collapse multiple named ' 
+                            'branches'))
+
 
         # Rebase
         if not targetancestors:

--- a/tests/test-rebase-collapse.t	Sat Jul 16 15:24:28 2011 +0300
+++ b/tests/test-rebase-collapse.t	Mon Jul 18 22:58:21 2011 +0200
@@ -442,3 +442,41 @@
   D
   F
 
+Interactions between collapse and keepbranches
+  $ cd ..
+  $ hg init e
+  $ cd e
+  $ echo 'a' > a
+  $ hg ci -Am 'A'
+  adding a
+
+  $ hg branch '1'
+  marked working directory as branch 1
+  $ echo 'b' > b
+  $ hg ci -Am 'B'
+  adding b
+
+  $ hg branch '2'
+  marked working directory as branch 2
+  $ echo 'c' > c
+  $ hg ci -Am 'C'
+  adding c
+
+  $ hg up -q 0
+  $ echo 'd' > d
+  $ hg ci -Am 'D'
+  adding d
+
+  $ hg tglog
+  @  3: 'D'
+  |
+  | o  2: 'C' 2
+  | |
+  | o  1: 'B' 1
+  |/
+  o  0: 'A'
+  
+  $ hg rebase --keepbranches --collapse -s 1 -d 3
+  abort: cannot collapse multiple named branches
+  [255]
+