Final answer:
Excel displays a small triangle in the upper-left corner of the cell when a formula fails to pass a rule check, indicating a potential problem with the formula that needs investigation.
Step-by-step explanation:
When a formula fails to pass one of the rules and background formula checking is enabled, Excel displays a small triangle in the upper-left corner of the cell assigned the formula. This indicator is a warning or error flag that tells the user there is a potential problem with the formula in the cell. When you see this triangle, it's a prompt to investigate and correct the formula if necessary. If you hover over the triangle, Excel will typically display a tooltip with more information about the issue.