Mine is crystal clear. I was four years old and we were at our traditional Thanksgiving Day parade. I was having a good time when all of sudden, a giant Baby Huey balloon turned the corner and came down the street. I was so scared and traumatized that I immediately wet my pants. My mom said, "What, are you a little baby who needs diapers?" Yes, that's all it took.
Okay, just kidding, but the thought was to good to pass up. I do have some personal conjecture. I was adopted at the age of 2, and I think I may have spent some time in an adoptive agency orphanage. This would have been the late '40s into 1950. I'm not sure what may have transpired as potty training.
I do remember my light bulb moment, the exact moment when I knew I wanted to be back in diapers, and I was 4 years old. It was after supper and I was running around the house saying da-da, over and over again. My mom had told me to stop talking like a baby a number of times, but for what ever reason, I continued. Finally she told me that if I didn't stop and start acting like a big boy, she was going to put me back into diapers, and if I thought she was kidding, she still had my diapers.
I remember really wanting to be in diapers and plastic pants, but my parents where having company over that night, and I didn't want to be embarrassed, wearing a diaper, so I said no, and stopped saying da-da. The thought of wearing diapers however, never left me. But like Trevor said, everyone has a different associative story, something I find very interesting.