Somehow, we didn't get keys to the inside lock when we moved in. This resulted in my husband locking me in 2 times once I arrived. Luckily, he was able to come home and let me out and, eventually, our landlords gave us the indoor keys. At night, we lock ourselves in, and leave keys in the foyer (but not reachable through the mail slot by any means short of magic).
The double whammy is that our door also locks automatically when it closes, so you can easily lock yourself out, too. We both learned that the hard way. I answered the door and took a step to receive a package and SLAM, there I was, locked out in my PJs. My husband had done the same thing, but he managed to get back in using a wire hanger through the mail slot to catch the door latch. Handy for him and a good reminder for us of just how easy it is to get in.