I grew up in the same house my whole life using the garage door as an entrance and therefore never grew accustomed to locking doors behind me. When I first moved out of my parents' suburban home that I grew up in and into my now big city apartment, it took me a while to remember. More times than not, I would stumble home drunk, get distracted and just forget.
I might add my apartment unit's overlooks a relatively secluded dumpster alleyway. My front door is outside of the main building. There are a lot of halfway houses on the block below me, and I've seen a few people doing drugs outside my window before.
One night around 2 or 3AM last summer, I was home in bed laying awake because my kitten at the time was loudly wrestling with my friend's puppy that I was babysitting for the weekend. I hear the door open, and see the shadow of a man walk into the hallway and just sort of stand there for a second. The dog was REALLY small, but stayed in my room and started growling at the intruder. He turned around, and walked hastily out the main heavy door.
Had the dog gone around the corner and revealed she was Jack Russell sized, or not been there at all, I could have seriously been royally fucked. The scary thing was earlier that night when I had some friends over, someone else had just walked right in, apologized for the mistake, and left. He could have been returning.
And I never forgot to lock my door again!
TL;DR: A man walked into my apartment when I was home alone at night. The puppy I was babysitting scared him away.