For short-term investments, the expected cash flows are most critical to estimate well (see Section 4.1A on page 70). In this case, the trouble spot

(d) is really all that matters. For long-term projects, the cost of capital becomes relatively more important to get right, too. The market betas and risk-free rates are usually relatively low maintenance (though not trouble free), having only modest degrees of uncertainty. The equity premium will be the most important problem factor in the cost-of-capital estimation. Thus, the trouble spots for long-term projects are

(b) and (d).