When designing for a large project, there are certain pages or elements that designers frequently overlook during the design process. Many of the points raised are made by beginners and/or in projects where there was not enough time to create wireframes to anticipate all the flows and problems.
The onboarding screen is a guide, intended to provide a brief introduction to the application, its main functions, and how to use it. It’s sometimes known as first user experience (FUX).
For pages that no longer exist, the URL entered is incorrect, or the link leads to a dead-end, etc. The site must lead you to 404 pages. You need to design it or else it will cause the user to lose interest in your website/app.
At times when the user’s device faces connection issues, the UI must display a screen to tell them about the trouble of connection, like an empty state, error, this is an issue that disrupts the user experience.