Despite the e-brake being set, my car rolled into the street yesterday

So yesterday after I drove home from the office, I tried parking my car in my driveway, backed-in. I've done this hundreds of times. My driveway is on somewhat of an incline. I got my car positioned where I wanted it, so I put it in park and then released the brake. As soon as that happened, it started rolling down the incline and I could hear a clicking sound, I assume from the transmission. It took several tries pulling forward and backing up to get the parking gear to engage fully. That said, I didn't want to take any chances, so I also engaged the e-brake.

About an hour later, I left the house with my son in our other (family) vehicle to take him to his dentist appointment. Shortly after we arrived (maybe 30 minutes later), while I was trying to fill out his paper work, I got a panicked call from my wife, explaining that my car had rolled out into the street. It fortunately didn't hit anything or anyone. She was able to recover my car and park it in front of the house. When I got home, I moved it into the garage.

I guess I can't park on any inclines for the time being.