Well, as per above, these are extremely complex requirements, so most don't make for a good story.

One of the simpler examples is that a customer wanted a solution for connecting special hardware devices across the globe, which are normally only connected directly.

Then, when we talked to experts for those devices, we learnt that for security reasons, these devices expect requests to complete within a certain timeframe. No one could tell us what these timeframes usually are, but it certainly sounded like the universe's speed limit, a.k.a. the speed of light, could get in our way (takes roughly 66 ms to go halfway around the globe).

Eventually, we learned that the customer was actually aware of this problem and was fine with a solution, even if it only worked across short distances. But yeah, we didn't know that upfront...