If you choose Google Code and then enter a valid name, the inoperable URL box is filled and a green check mark appears.
If you then switch to Mercurial, the URL becomes editable and green check remains.
If you then switch back to Google Code, the URL box becomes inoperable again but the contents disappear. The green check stays.
Switch back to Mercurial, and the URL box is empty and editable with a green check.
Why was I doing this? Because I couldn't figure out the interface, was trying to paste in the URL I'd already copied, and presumed that the Name was what I wanted to call it on Bitbucket. I selected Google Code because that was one of the options, and then was confused as to what it was doing.
Suggested solution: Remove "Name", get rid of "Source", and just letting users paste in a URL and select a repository type. You could probably even autoselect the repository type. Users who are importing repositories can be expected to know how to copy a URL.