Final answer:
When a formula fails to pass one of the rules and background formula checking is enabled, Excel displays a small exclamation mark in the upper-left corner of the cell assigned the formula.
Step-by-step explanation:
When a formula fails to pass one of the rules and background formula checking is enabled, Excel displays a small exclamation mark in the upper-left corner of the cell assigned the formula. This exclamation mark serves as a warning to indicate that there may be an error in the formula. It helps users easily identify and fix any mistakes in their formulas.