custom pagers will not return correct order from view if your view sort criteria is not configured to apply to all displays

If, when you set up your view, your view sort criteria is set to "apply this page only", custom pagers will not work properly.

This is because custom pager code adds a new display to the view to generate the order results for the pager, and if the view sort criteria is not set to "apply to all" this new display doesn't inherit that sort criteria.