Available Fields
The "Available Fields" section in "Form Layouts" does not show all available fields. It seems to only show five fields. With no easy way to reach the other ones.
I've removed most fields from my default layout and added a couple of new custom fields, those fields are not visible unless I add most of the previous fields first (when I've added my custom fields I can remove the unwanted ones again).
It's no biggie now that I know how to work around it, but quite confusing at first.
Tested in
WP 2.9.1 and
WP 3.0b1