I was in the bathroom a few days ago when I started getting hard. I decided to go for it and started stroking myself. I was just about to cum when my sister banged on the door and said she needed to be in there. This startled me and caused me to miss the toilet and cum on the floor. I thought I had it all wiped up, so I left and she went in and closed the door.
I went downstairs and a few minutes later I heard her yell out 'eww, gross!' I knew what had happened, and a few seconds later she flew down the stairs. She only had one sock on and said 'clean up your cum next time, it's disgusting! You're lucky I wasn't barefooted!' She took off the other one and threw them at me and went back upstairs. I looked and there was a big wet spot on the bottom of it. I don't think I've ever been more embarrassed in my life.