Browser: Iceweasel 3.5.8

Steps:

1. Navigate to http://prototype.wikimedia.org/s-6/index.php?title=Main_Page&action=edit
2. Click in the textbox
3. Click insert link button
4. Put http://prototype.wikimedia.org/sandbox.6/Portal:Contents in the first field. To an external web page is selected automatically
5. Click insert link. I'm prompted The URL you specified looks like it was intended as a link to another wiki page. Do you want to make it an internal link?
6. Click Internal link
7. The insert link dialog is on the screen now. Text in two fields is changed. To a wiki page is selected
8. I cannot change neither of the two fields.
9. Click on To an external web page
10. It doesn't seem to be selected, but the text (and icon) at top-right changes from "Page exists" to "External link"
11. Click insert link. An internal link is inserted

The bug appears in step 10.

Version: unspecified
Severity: enhancement