When working with Revit we get the Warning message that pops up, similar to the example below (this one was due to 2 door marks having the same number).
Most of the time we may have a reason for ignoring the warning, other times we may have just clicked away from the warning or had it disappear from the screen. The good news is you can easily review these warnings at any point. To do this go to the following pulldown: Tools>>Review Warnings
If your Review Warnings is greyed out it means you have no warnings at this time. This can be a good opportunity to review projects that are underway to make sure there are no issues that need attention.
Happy Revit'eering...