multiple heads

[Neal Becker]

 hg heads
changeset:   636:ca6d8a69cb5e
tag:         tip
user:        ndbecker2 at gmail.com
date:        Thu Mar 19 09:53:54 2009 -0400
summary:     Add +incdir to SConstruct

changeset:   632:f927fd11da2c
user:        ndbecker2 at gmail.com
date:        Mon Mar 16 11:02:17 2009 -0400
summary:     run-tests-beta.py: init

changeset:   555:1844d569dbb3
user:        ndbecker2 at gmail.com
date:        Tue Feb 10 11:21:14 2009 -0500
summary:     merge changes from nbecker6

 hg merge 555
abort: outstanding uncommitted changes

 hg diff -g
[silence]

hg status -m
[nothing]

Now I'm really confused!  What are these uncommitted changes then?