I am not really the party type. I tend to stand near the food and hope someone talks to me, especially if my husband has wandered off to talk to other people.
When a masked man enters.
Probably one of the superheroes has arrived presaging imminent disaster. Or it could be a highway man who had lost his way but still wants your valuables. Of course if it is Carnevale in Venice, just party on.
When the dancing starts.
The music will be too loud and most of the dancers will be really bad, at least the ones who want to dance with you. On the other hand, if you are in the Assembly Rooms in Bath with Jane Austen, you might want to stay and see if Mr. Darcy asks you to dance, assuming you pass muster.
Your ex walks in with his hot new girlfriend.
How embarassing if you are alone. But if you are there with Clive Owen and Matthew Macfayden dancing attendance on you, your ex will probably slink out instead, no matter how hot his new love happens to be, unless she is Angelina Jolie. But Brad might object to that.
Your boss walks in.
If this isn't an office party you will feel uncomfortable, especially if you have had a lot to drink and tend to speak your mind. You might stay if you and your boss are friendly socially but I haven't had that experience very often.