Need to examine IRR assumptions to see where they don't hold: 1. IRR assumes cash flows can be re-invested at same rate (IRR) through life of project. 2. IRR assumes cash flows occur at regular intervals throughout project...in practice you need to specify an interval and populate the cash flow for each period (including zero) if there was none. 3. No closed-form/analytic solution for IRR. If the cash flows are highly irregular the root finding algorithm employed to find IRR might find more than one root.