Once, I had to specify HMI of air traffic controllers for a simulation. The controllers have described their hmi in meetings, then I have sent them the document

Once, I had to specify HMI of air traffic controllers for a simulation. The controllers have described their hmi in meetings, then I have sent them the document to review, then I went to an ATC center to watch controllers. I was amazed by the number and importance of differences between my reviewed document and reality. Many details that seem obvious for the final user may cause drastic changes for the developer.