A tree is on the power line / there are sparks outside — what should I do?

A limb or trunk lying across a wire, a span sagging much lower than its neighbors, arcing, buzzing or popping sounds, scorched or smoking bark, lights browning out or only part of the house having power. A conductor can also be lying in wet grass looking completely inert. The tree has become an electrical conductor. Green wood and wet bark carry current, so the tree itself, any fence or gutter it touches, and the ground in a radius around a downed conductor can all be energized. Until the utility says otherwise this is an electrical incident that happens to involve a tree, not a tree job that happens to involve a wire.