For example, already incurred costs like preliminary consulting fees should not be included in the analysis.
For example, negative impact of a new diet soda product launch on the sales of existing soda products.
(Note: In a conventional cash flow, the sign of cash flows changes only once during the life of the project; while an unconventional cash flow has more than one sign change.)
For example, if you plan to use an existing office space rather than renting it out, then rental income from the office space is an opportunity cost.