I have tried to use one managed metadata term set for global navigation for all of my site collections and failed with the following error showing up at some point after repeated editing of the term set: Error loading navigation: The Managed Navigation term set is improperly attached to the site.”
First, I created a master term set in the global term store with the global navigation choices and then I selected that term set for the root site collection navigation. Second, for all other site collections, I created a new term set in the local (site collection) term store and I pinned the terms from that master term set as children. This worked fine for the other site collections but the root site collection that actually used the global term set for navigation without pinning was the one that failed with this message: Error loading navigation: The Managed Navigation term set is improperly attached to the site.”
Noticing this, I tried again and the key is to keep and maintain that master term set as a template, or source, to be used for pinning but do not choose it as the term set for navigation on any sites or collections. You can change it but don't use it for anything else except as a source to pin term with children. Then when you want to setup navigation for any collection, create another term set in the local (site collection) term store and then"pin term with children" from the master term set.
TIP: It is best to get this master term set figured out as much as possible and defined with all of the links before pinning because when the master set is changed then the best way to cascade this down to the pinned sets is go to the site collections where it is pinned and delete the term and then re-pin the term so that the changes show up.
Is there a way to get the changes to the master term set to show up in the pinned terms? The titles change but the actual links and settings do not.