I didn't think they knew what I was doing, until it was too late.
Once, when I was about 11 or 12, our family was in the family room watching TV. Something wholesome, but I don't remember. I was lying on the floor on my back with a blanket over me. There were 3 other kids in the family, 2 brothers and 1 sister.
I don't know what possessed me, but I decided to finger myself under the blanket. I tried not to go too fast, or make too many obvious movements. I'd die of embarrassment if I got caught! After about 5 minutes, I was in my own world, fingering my clit and started going faster and faster, almost forgetting where I was. My breathing became heavier, and I was so close to orgasm.
All of a sudden, one of my brothers asked what I was doing, and although startled, I just looked over at him, and couldn't stop what I was doing, because I was at the brink, and actually started going faster and faster to finish what I had started. I realized that the whole family knew what I was doing, as they just stared in wide-eyed amazement, but I just couldn't stop. Within about 8 or 10 seconds later, I came very hard, and could not stifle the 'ooohhhhhhhhh' from my mouth, and my whole body just started to shudder uncontrollably. At that point, nobody can stop what they're doing, so I just let it ride out, and became limp afterwards.
I was so embarrassed, that I got up and as I was running out of the room, I heard my brother snickering, saying something like 'She's going back for more', and then my mother telling him to shut up. I stayed in my room the rest of the night, and it was never brought up, but to this day, I realized that my whole family saw me masturbate in front of them, and actually come! Even though it was embarrassing at the time, I still use this image in my head when I need to masturbate and having a hard time coming.