Much like earthquakes or Voldemort attacks, running into your ex happens when you’re not expecting it. No one ever really expects to run into your ex, unless you live in the same neighborhood and you’re in that territory where you’re not sure if it’s “your” area or “theirs”. Then you feel like it’s Halloween night, the hair sticking up on the back of your neck, as you look around every corner, wondering which spirits will jump out at you. Even if you had a good breakup, running into your ex can be jarring. Especially if you’re running into them with your car, which hopefully you are not, but hey, things happen. It’s Vegas.
How to handle these run-ins can depend on how your breakup went and what sort of mood they’re in when you see them. Only you know your relationship history so use some of your discretion when deciding how to approach. These are suggestions, not laws.
If you see them, and they see you, don’t pretend you didn’t see them. Unless your ex is an abusive jerk or if saying hi to them would be very upsetting for both of you, it is incredibly cruel and immature to pretend that you didn’t see the person whose tender arms you used to fall asleep in, or did hand stuff with, I don’t know your relationship. Acknowledge them. Sometimes all you need to do is wave. But if it’s the kind of situation where you can go up to them and say hi, you should.
Be mature. Be confident. Be brave. If you can go over there, go over there. Say hi. Smile. If you’re on good terms, hug them hello. If that would be weird, then don’t.
Keep it brief. Especially if you’re with another person, and especially squared if that other person is someone you’re seriously involved with, or just dating, or just doing hand stuff with, I don’t know your relationship. Your goal here is to be a human being and say hello to a human being you used to have a history with. That’s it. You go over, you say hi, you ask them how they are. They’ll ask you how you are. You’re both fine. Say something nice. If you’re with someone, introduce them. Don’t just leave them standing there. If they’re with someone, they’ll introduce their someone. Say it was good to see them. If you’re at a bar or restaurant, suggest something good they could try. It’s about being friendly. Then say goodbye.
Don’t compliment their looks. Even if it’s meant in a totally innocent way, don’t do it. Unless they lost 50 pounds and you know it was something they’ve always wanted to do and they’re proud and you’re proud of them. Otherwise, don’t. It can get awkward fast.
You’re in control; it’s only awkward if you want it to be. But if it DOES get awkward, don’t linger on it. Acknowledge the awkwardness with a laugh, and move on.
Don’t let it ruin your night. If you’re not on the best of terms with this person, it’s easy to let this destroy your day or night. It doesn’t have to. Some of us will see our ex and then see a martini, 3 times. Try not to do that. You’re human, and that’s okay, but don’t let anyone turn you into a sobbing, drunken mess, because it’s not worth your energy, time, health, or money.
If your ex runs into you, and they’re mad, don’t fight back. Let them say what they want to say. Do not help them make a scene. Do not give them anything to run with. Let them have their tantrum, tell them you’re sorry they feel that way, you hope they feel better, and then leave. If you feel you can handle it and you really care about them, invite them to talk about it over the phone or in person when they’re calmer. But you don’t owe them anything.
When you think about it, really think about it, it’s strange that two people go from spooning and crying and laughing and hugging and holding and sharing to, “Hi, how have you been?” But what else can you do? 
