You might have this fantasy that the person who you passionately loved when you were younger or you met briefly at some other point in your life and don't know what happened to them is the one who got away and that someday they'll come back and it'll be this absolutely perfect love story.
Personally, I blame The Notebook for this notion (don't get me wrong - it's my favorite movie), but it's a very rare and unrealistic thing to happen. Can it? Sure. But sometimes the past should stay in the past and you don't have to wonder what if or ultimately choose them in the end.
You might find yourself thinking about this person and the time you shared together. For one person, it was a one-night meeting where you had the time of your life and then each parted ways to go back to your lives. For others, it was a long relationship that life drifted or even tore you two apart, and you always wonder what could have been. To relive it, you might play songs that take you back to that time or visit places you two experienced together or stare at old photos wondering what happened to the people in them. It's okay to not go back in time and look at old photos or listen to old songs and feel nostalgic. Sometimes, it'll mess up your present state of mind and can ruin things because you want a taste of the past or a person in your past who doesn't necessarily fit in your present.
Let me ask you this: is the one who you believe got away different, or are your lives all of a sudden aligning to where you think it'll truly work out this time? If this is a yes, I truly hope you found your happy ending. If you're unsure, however, I don't think it's a bad thing to say no to this person coming back into your life.
Sometimes the past is better left in the past. Each experience and relationship can truly be appreciated and loved for exactly what it was, no matter how much you might wonder about what could have been. You can look at it and see what it taught you and how much you loved being with that person in that time, but not let those feelings and uncertainties leak into your life.
If you have someone in your life or your past who you consider the one who got away, it probably happened for a reason, and it's okay to not question that. You may realize that reason was to teach you a lesson about moving on, to push you to find a new job, live in a new place, or find your soulmate - or you may never know the reason they got away. Regardless, they got away for a reason. It's okay to move past them, it's okay to move on. Your life is where it is because you experienced that heartache or had to make that decision, because they are no longer a part of your life, because they got away.
It's okay to love what you two experienced and had, but it's okay to not choose them in the end.