I can add up to 17 items to my Menu. As soon as I add one more item by clicking the checkbox for the page that I want to add in the Pages box in the left hand column and save the menu, the item that is last in the list drops off.
The Appearance Menus Screen page in the Wordpress codex makes no mention of this limit.
I am using a child theme of the Thematic theme; but I have had this problem before with other themes too.
If there is a limit, is there anything that I can do to override this?