Occasionally, you might stumble upon a strange error, that happens both on production servers and dev server (ie with the php dev server). You might try to edit (enable/disable/reorder) a menu item in a Drupal menu, but this won't be saved.
This could happen on really big menus, with a lot of items in them. PHP is blocking this change, as a security measure!
For people not running the Suhosin patch, editing the following value in php.ini should do the trick:
In my case I had to bump this up to 3000!