This is not always applicable. There's always the classic advice of "When in doubt, have a man come through the door with a gun in his hand." (Debra Doyle observed at Boskone that the general case is, "Introduce an unsettling element" like an invitation in a Regency that is from someone where the question is why would it be sent? But often enough, it's action.)
The thing you have to remember is that you are filling out a check when you throw it in. You will either need to cover the amount, or tear up the check. If it doesn't fit into the story, it's going to have to go out. Likely, to fit, it's going to need support before and after the point at which you introduce it.
Though it works best when you don't know where the story's going. Once you do, you're either going to have to fit the new element into the ending as well, or decide that's impossible and throw away the ending.
