It seem there is some bugs with the assigned to attribute.
In the database it seem it's linking using 'SQL Text' while it should link to user id 'SQL Integer'.
Because of that changing the name of a user result in the lost of all issues linking to this user.
I had to re assign all of my bugs :/ was a boring task: there was around 100 bugs.
Please change this meaning. I had the problem in the web interface, but i assume it should be same for desktop interface.