My organisation, which provides information and advice, is starting to sell things online as well. The 'shop' will be run on a different platform and is effectively a different website (but will be branded to look like our main website).
Our current website's global navigation (horizontal tabs) is topic-based, eg 'Money advice', 'Health advice' etc. We want to add 'Shop' to the navigation. But if the user clicks it, they'll be taken to the retail site, which has different horizontal global navigation (eg 'Kitchenware', 'Gardening' etc).
I feel like this breaks the 'tab' design pattern - if you click a tab, you don't expect the rest to disappear.
But if we change the design of the shop enough, it should be clear the user has been taken to a different site - so will they still be confused?
A similar example is http://www.redcross.org.uk/