Warning messages appear before we ask people to take action, usually in anticipation of a significant change. A warning message needs to leave your readers feeling:
Consider the task the user is trying to get done and how this message will impact this.
Think about how the user is feeling at this point in their journey.
Make sure you're not talking about something that has already occurred (this should be an error message).
Use concise, scannable language that communicates the problem at a glance.
Always give the user a clear action to keep moving forward.
Have empathy for the user. Inform, but don't alarm them. If the warning comes before an action, clearly communicate what will happen if they proceed and provide an alternative where possible.
Be practical. Depending on the context, be careful of using wink here or too much optimism.
This flag clearly states what the issue is and gives the user two clear actions to move forward.
Banners are an effective way to warn people about issues or an impending change to their experience, without being intrusive. Each message is visible at the top of the screen, and the language is clear about what’s happening and what the user can do next.