Remember one thing i.e to use __ (double underscore) after package name. Suppose you have a Visualforce page that uses custom controller. Visualforce page name is Upload_a_File. You have created a button on this page to choose a file and then upload this file to Amazon S3 or some other location.
URL of this page is : /apex/Upload_a_File
Everything works fine in your developer org. The problem comes when you create a managed package and install it in a destination org. When you press the button, an error message shows up :
‘Page Upload_a_File does not exist’
However, if you enter following URL directly into the browser, page successfully shows up:
Reason and Solution:
Namespace prefix is not automatically pre-pended for the URL specified behind click of the custom button, which is the reason for above error.