Mercurial wrong "abort: can't merge with ancestor" error

Binary package hint: mercurial

Suddenly got an error:
$ hg merge release-2.4
abort: can't merge with ancestor

As you can see on the screenshot no one branch is an ancestor. Branch release-2.4 was closed using hg commit --close-branch.

Merging using explicit revision (hg merge 2005) number works (though with strange conflicts), reverse merge (hg checkout release-2.4; hg merge default;) works too.

I don't understand what has happened.

ProblemType: Bug
DistroRelease: Ubuntu 10.04
Package: mercurial 1.4.3-1
ProcVersionSignature: Ubuntu 2.6.32-25.45-generic 2.6.32.21+drm33.7
Uname: Linux 2.6.32-25-generic i686
Architecture: i386
Date: Tue Nov 9 20:41:40 2010
InstallationMedia: Ubuntu 10.04.1 LTS "Lucid Lynx" - Release i386 (20100816.1)
ProcEnviron:
 LANG=ru_RU.utf8
 SHELL=/bin/bash
SourcePackage: mercurial