When you create a custom block, neither the title nor the body are required, only the description. Blocks without content don't show up - which means you can put the empty block in a region, and there's no indication of why it's not there.
Some users when creating a menu item (either from the node form or in the Menu administration pages) will select the default "Add a main menu item" item from the list of menu items.
Implications:
Without the menu items at all, users also get confused about where the "Main menu" and "Secondary menu" exist.
The "Add a main menu item" item should not be a real menu item. Instead it should be added through the theme layer when the menu is being displayed and the menu is empty.
Users are often confused by the large number of items placed in the menu before they have even added any items. They often wonder why they can add an item to the administrative section at all.
Implications:
Users also get thrown off by items like "Main menu" being displayed in two places: as a root level item and under Admin -> Site building -> Menus -> Main menu.
