About once a week, I have this scenario. One of my clients is supporting ebook software. Most people login to their library (if online) through the primary website, but some people will login through publisher-specific portals. If they login through the latter, they only see the publisher's books.
Makes sense.
So about once a week, at least, I get this:
User: 'If I login to the app on my tablet, I see all my books. If I login to the website, I don't see <specific title>.'
Me: 'Are you logging into <our link> or <publisher specific link>? Please send a screenshot of what you're seeing.'
User: 'I always login at <our link>! <screenshot clearly shows publisher specific link>'
Me: '...You're logging in at <publisher specific link>. If you login at <our> link, you can see every book. Please try this.'
Usually they won't email back. A few especially stubborn ones will, trying to argue that I am wrong and just trying to make them look bad. I don't know why they don't just, from the very first email, go 'oh let me try this other link mentioned and see what happens.'