When I was in my first full time job, I resigned to take up a new role. In the interim period I started to really second-guess my decision but I went through with it anyway because I was too embarrassed to go back and say I'd changed my mind after they'd already announced I was leaving. Surprise surprise, I didn't like the new job and ended up leaving there really quickly!
I'm now at a point where I'm considering resigning from my current job for entirely different reasons and thought of that one occasion from years ago, and it's really niggling at me! I had a colleague at a different workplace who left her job, started a new one and came back within a month previously so I know it happens. It was probably a bit difficult for her to do but our boss was pretty good about it.
Have you ever resigned and then changed your mind?