Financial additionality and viability for Clean Development Mechanism (CDM) projects is commonly demonstrated through deterministic internal rate of return (IRR) benchmark analysis, supplemented with a sensitivity analysis. This method is vulnerable to assumptions on cash flows (including that from carbon credits) and is unable to differentiate grades of separation from the benchmark. This paper examines the financial additionality test and viability in the presence of cash flow uncertainty, where IRR becomes a random variable. A case example project involving wind power is exampled. It is seen that the boundaries between acceptance and rejection as a CDM project based on financial additionality and viability tests become blurred, leading to possible alternative conclusions. Comment is given on false positives and false negatives, in terms of acceptance and rejection of projects.