Here is a suggestion, taking three steps. It uses conditional
formatting, which you may or may not be familiar with.

Wherever the cell contents of sheet1 and sheet2 don't agree, the word
"Error" will be displayed in red.
Wherever the cell contents of sheet1 and sheet2 do agree, the cell
value will be displayed normally.

Step 1.
In sheet3, cell a1, type the formula:
=IF(Sheet1!A9="","",IF(Sheet1!A9=Sheet2!A9,Sheet1!A9,"Error"))

Step 2.
Use conditional formatting:
Click on cell a1 of sheet3.
Click on "Format", then click on "Conditional Formatting"
In the Conditional Formatting window, choose the following:
"Cell Value Is", "Equal to", then in the 3rd box  type ="Error"
Click on the "Format" button
o For Font Style, choose Bold
o For Color, choose red
o Click the OK button
Click the OK button again

Step 3.
Copy and paste cell a1 into as many cells as you need.

Hope this helps. Of course, you may choose other format options, or
choose to display something other than the word "Error".