--- a/hgext/rebase.py	Thu Oct 08 10:07:41 2009 +0200
+++ b/hgext/rebase.py	Thu Oct 08 10:39:43 2009 +0200
@@ -401,6 +401,10 @@
             return None
 
         cwdancestors = set(repo.changelog.ancestors(cwd))
+        if dest in cwdancestors:
+            repo.ui.debug('source is descendant of destination\n')
+            return None
+
         cwdancestors.add(cwd)
         rebasingbranch = cwdancestors - targetancestors
         source = min(rebasingbranch)