I've never been caught, but damn I almost was one time when I was about 14 or 15.
Going through puberty you do stupid things sexually, and this would be one of those instances. I had printed out a few random porn images off the PC so I would have something to whack to for later.
My sister and my mom were both outside, so I figured this was my best opportunity to strip down and take care of business in the bathroom. Sitting on the floor completely naked I distributed the multiple printouts around me and got to work. I took my time as I knew my sister and mother would be outside for a while.
Suddenly, I heard someone coming running in the front door shouting: "Brandon! Are you in the bathroom?! I've gotta come in for a second!!" The door knob started to turn.
Fast as lightning I gathered up all the images in one swoop, along with my clothes, and shoved them in between the cabinet and the toilet. I jumped up and sat on the toilet. My sister raced to the sink and started washing the blood off of what was apparently a pretty bad cut on her finger.
So there I was sitting nude on the toilet with multiple porn printouts bunched in the corner next to me, with my sister's back facing me. Lucky for me our bathroom was setup so that whoever came in would be facing the sink, and have to turn around to look at the toilet.
It wasn't too long after that that my sister started crying, and of course my mother came rushing in the bathroom as well to look at it. Now both of them were standing there at the sink. Finally my mother said, "Here, let's go out to the kitchen sink so your brother can finish using the bathroom".
I really think the masturbation gods were looking out for me that day, because either of them could have easily just looked in my direction and seen me naked with porn.