Your End Game. When you hand in your resignation, it can kick off a bit of chaos__your position will need to be filled, but your duties will also need to be covered until they can replace you. As part of your resignation letter, it's a good idea to convey what your availability is during that transitional period.
Spot the Signs. According to Alison Green, who writes the blog Ask a Manager, this is common practice for some industries, especially if you're leaving to work for a direct competitor. However, in those situations, you'll likely know it's coming and can prepare accordingly.