Data validation is designed to display messages and prevent invalid entries only when users type data directly in a cell. Validation messages won't appear and invalid data can be entered when a user enters data in a cell by copying or filling, a formula in the cell calculates a result that isn't valid, or a macro enters invalid data in the cell.
You may want to audit your worksheets to look for incorrect data that may cause inaccurate calculations or results. You can identify cells with data validation that contain invalid data by displaying a red circle around them so that you can easily find and correct any problems.
What do you want to do?
Circle invalid cells
On the Data tab, in the Data Tools group, click the arrow next to Data Validation, and then click Circle Invalid Data.
Microsoft Office Excel displays a red circle around any cells that contain invalid data.
All cells that don't meet their data validation criteria are circled, including values that were typed, copied, or filled in the cells, calculated by formulas, or entered by macros.
Hide validation circles
Do one of the following:
To remove the circle from a single cell, enter valid data in the cell.
On the Data tab, in the Data Tools group, click the arrow next to Data Validation, and then click Clear Validation Circles.