Can confirm, about 10 years ago, the company I worked for migrated to AWS, and I managed the transition. We planned everything meticulously so that there would be no downtime, and used it as excuse to fix a lot of tech debt. No one was supposed to even notice the cutover, and when we did it, I expected the only feedback to be that things seemed faster and were working as expected. A few hours later, we get a complaint from an Account Manager for Walmart that they can't access the platform at all. There was a lot of confusion and back and forth, turns out their IT department had an allow list or something in the corporate DNS to not resolve to AWS owned IPs unless approved. We eventually got them to add our domain to their allowlist, but it seemed insane that they would spend the effort to implement and maintain that level of control.