I don't know quite how to describe it, but it was the most unique emotion/feeling I've ever had. And I'm not sure if I liked it or hated it. It was some sort of combination of sympathy, shame/guilt, embarrassment, joy, laughing(happy?), and amusement. Simply put, it was kind of like seeing someone do the same cringe-worthy embarrassing things that you used to do, and you all you can do is laugh because you know exactly what they are going through and what they are feeling. It's the only emotion that has ever made me physically react (besides moving my face). It was both sad, but amusing. And the only thing I could do is cringe and laugh at the same time, two things that usually never go together. Edit: I just noticed your question: Before, it's more like anxiety. You know what's about to happen, and it doesn't look good. During I described, sort of a cringe with a realization that you've done it before, then I don't quite know what happens but it's pretty hilarious because you realize that you understand the person's situation exactly. Afterwards I kind of feel a built guilty because I laughed, but sympathetic. It's hard to describe.Describe the experience before, during, and after it happened.
I have a similar experience when I watch someone act in a way that I know I used to act, but I'm able to see it from an outsider's perspective and I go through a mix of shame for being such an uppity bonehead, happiness that I'm not the only one that does it, sympathy for the fact that the person I'm witnessing is going through it, and a nice little release from my seriousness. Put more simply: being able to laugh at yourself when you see someone do something you used to do and you can see it from the perspective that those around you saw it (particularly older)