This happened in my second year of high-school. I was working on a project with a cute girl in my class. We had flirted a few times before but nothing too involved.
I was interested and I was sure she was too, so I planned out the perfect scenario. I purposefully left the project until the very last night when I knew my entire family would be out.
Fortunately she had a penchant for procrastination as well, so my sneaky plan didn't appear obvious to her. I told her that I'd be free at 5 when I got home from a lacrosse practice. I made it seem like I would be in a rush to get home in time, which gave me the perfect excuse to be in the shower upon her arrival. I told her to just let herself in if I still was.
She arrived on time at 5, just as I was hopping out of the shower. I heard the door open and casually strolled out of the bathroom onto the upstairs balcony wrapped only in a towel. As she was taking off her shoes in the front lobby I looked down at her and smiled. Without saying a word I walked across the balcony into my bedroom. I remember not having heard any footsteps so I assumed that she must have been just standing there. That's when I called down "so are you coming up?"
We never finished the project.