This happened to me several years ago at the apartment complex I used to live at. I was living alone at the time in a building with about 100 other tenants. One of those tenants is a 33/34-year-old woman named Cheryl who lives across the hall from me. She's about 5'6', 120 lbs, short sandy blonde hair, brown eyes. She is really beautiful. She's a professional, working woman (based on how she dresses) and she has a live in boyfriend.
The building has a parking garage and my assigned spot is on the third floor, hers is on the second. I hopped on the elevator one afternoon with a couple of bags of groceries and she hopped on the same elevator a floor higher and we shared the elevator up to the third floor.
She was wearing the most wonderful perfume. Couldn't figure out what it was. Anyway, the smell plus her outfit (sophisticated, professional) plus the fact that she is beautiful got me hard. She walked out of the elevator first and was about three paces ahead of me the whole way to my apartment. She has the most beautiful body and the sexiest ass.
I got to my apartment door, put down my groceries, unlocked the door and entered my apartment. I put the groceries away, but I could still smell her and I was still very aroused. So I did what most men would do in that situation.
I sat on the couch in my living room and started to remove my clothes. After I stripped down naked, I began to masturbate, fondling my erection and balls with my right hand while my left hand caressed parts of my body (my nipples, inner thighs, etc.). I wound up lying on my back on the floor thrusting my pelvis in the air into my hand and I started to moan, every once in a while letting out Cheryl's name. As I got closer to orgasm, my moans got louder and Cheryl's name became more prominent. At the moment of truth, my door opened and Cheryl walked in! What she ended up seeing was me on the floor shooting cum all over my belly as I was calling out her name. She knew she walked into something awkward and took a step back and tried to discreetly close the door behind her as she left.
The incident so embarrassed me that I tried avoiding her if at all possible. I might see her getting her mail or walking in the hall to and from the trash chute or the laundry room. Every time she would just smile, knowing she's got the goods on me. I moved about four months later and never saw her again. Since that day I refer to that position as The Cheryl Position and dream of her catching me again. The incident no longer embarrasses me. I find it an exciting experience that I hope will happen again with another woman in my fantasy life.