Take a look at the following article about the STSADM migrateuser command. The
problem has to do with the change in Security Identifiers (SID) that took place
when the user  was migrated.

technet.microsoft.com/en-us/library/cc262141.aspx

I have been using the -migrateuser command for other
issues, I just haven't done it yet with this particular user.

While we're on the subject of the -migrateuser command. I have noticed
that sometimes I get an error using the command and other times I get
a confimration message. It has been spotty and happens about 50/50 on
the two old AD domains I am migrating users from to the new AD domain
I am moving users to.

I have tried 'false' after -ignoresidhistory and other left it
blank--which defaults to 'false'. Yet, even with the error message,
the user  appears to have been migrated  okay.

Any ideas?

In this case I would recommend running it with -ignoresidhistory set to True.
The default is false which tries to verify that the new user  matches the old
user by comparing all the metadata for the user in both domains. The error
probably comes because something like Display name has changed as part of the
migration for some users. Setting it to true will still migrate the user, but
it will assume that you've actually chosen the right old and new user.