Many experts recommend against applying paint even at the slightest hint of rain. But if you live in a rain-prone area, this is usually not possible. Professional painters especially cannot afford to stop exterior painting every time a rain cloud comes onto the horizon.
Insects, automatic lawn sprinkler systems, direct hot sunlight, high winds and other local conditions can suddenly pop up and ruin a nice job. Choosing the wrong paint. Generally on the exterior, you can safely cover oil paint with latex, but not vice versa.