Diageo (LSE:DGE) has increased its dividend annually for the last 37 years, but its share price is down 14% over the last 12 months. Sales might be down, but the stock’s down more. As a result, Diageo shares trade at their lowest price-to-sales (P/S) ratio for 10 years. This looks like a stock market overreaction – and an unusually good potential opportunity. At its latest update, Diageo reported a 1.4% decline in revenues during the second half of 2023. The weakest region was Latin America and the Caribbean (LAC), where sales fell 23%. Management attributed this to a number of factors. These included unusually high sales in the previous year, consumers trading down, and a weak macroeconomic environment. Excluding these – as the company pointed out – Diageo would have achieved revenue growth of 0.7%. But while I understand the justification for exclusion, I’m not convinced by it. Fortunately, the case for thinking the stock is a once-in-a-decade opportunity doesn’t depend on ignoring the revenue decline. Even with this, the stock looks unusually cheap. Diageo’s share price has been falling faster than the company’s sales. As a result, its P/S ratio has fallen from 4.95 a year ago to 3.76...