Badly damaged or missing shingles. Gaps, gaps, gaps caused by shrinking/swelling cycles around chimneys, vent pipes, etc. Anything that looks at all suspicious for letting water pass though gets silicone or blackjack from me. In spring and fall, we can have extended rainy periods where I live, so I error on the side of caution. It's a good chance to review the overall condition of the roof, too since replacing it is a major expense.