Nothing let's you know that there is a problem better than a big red error message! The Campaign Designer has an Errors tab to easily shed light on problems in a campaign.
After you have added all of your campaign steps, if there is anything that is preventing your campaign from having a valid flow (meaning that there is something that will prevent the subscriber from being able to move on to the next step), the Errors Tab will appear.
Click the Errors tab to display the error details.
Once all errors have been resolved, the Errors Tab will disappear!