As we build a new SharePoint environment, I am told that we had a
problem with one of our old ones, so I don't want to repeat history!

They had a custom control to provide some common navigation links to
the sites in a site collection. The control would have a link (say)
called "Topic A" that linked to a particular page, and that control
with that link (among others) would appear on every page across the
site collection.

The problem was that the search crawler saw the anchor text "topic
a" on every page, so if a user searched on "topic a" then every page
in the site collection would be a hit, even though most of them are
irrelevant to topic a.

I am told that they eventually resolved it by putting some extra
code in the custom control to tell it not to render when hit by the
search indexer.

I would have thought this problem would occur in any web site with
common navigation links, including SharePoint portal sites with
navigation links to areas. How is this normally dealt with?