A user-defined menu item is not displayed on the device client.
Menus that are created on the client are already part of a table's context, so if you create a menu item with the same name as the menus that are already created on the client, they do not display on the client.
You can remove these menus by creating an empty one of the same name under the design, or you can overwrite the menu in the same context but have it perform a different function.
For example, if you create a menu called "Detail" and assign an alert action to it, the the action is selected, an alert displays saying "No details available" instead of bringing up the usual details panel.