convert from svn enters an endless (or possibly just very long) loop when the
conversion is completed. I'm testing with the following command:

hg convert --rev 139 --debug svn://svn.lyx.org/lyx/lyx-devel /tmp/convert-test

(Note: rev 139 is the first revision under lyx-devel; converting up to 138 
works fine --- i.e., does nothing, but doesn't hang.)

This did not use to happen, and bisect finds that it was introduced by this:

The first bad revision is:
changeset:   6392:5efd447a9b8d
user:        Patrick Mezard <pmezard@gmail.com>
date:        Sat Mar 29 17:15:45 2008 +0100
summary:     convert: follow svn tags history (issue953)

I first reported this to the mailing list about a month ago, there's a debug log
attached in the original message
(http://permalink.gmane.org/gmane.comp.version-control.mercurial.general/7131).
Only recently was I able to confirm this from a separate machine.