While beating it isn't an activity you normally "share" with others, let alone family, we all know that everyone does it, because as humans we all have thus urges. So I cannot for the life of me fathom why someone would get mad catching someone else doing it. I mean it is one of the most natural things TO do, like eating and sleeping (our 3 most basic instincts). It would be like if she got mad catching you eating or sleeping. I mean, I get it, getting angry is sometimes just a reflex when something REALLY surprises you and you don't know how to react. But then to go on an telling your dad about it, as if you genuinely did something to be scolded for. That's just a low blow (no pun intended).
Now that we are being honest with each other though, I was never ONCE caught during my teen years. It wasn't until... last year actually (so fairly recent), that my brother, whom lives in the apartment above me, did so. My door was unlocked, and while I am usually very aware of my surroundings especially during quality times, this time my headphones - and the fact I was so immersed on my PC - made it so that I didn't hear him this time (dude has a habit of barging in at the worst times). Like, he is the type who frequently gets the urge to check in on people, while I'm the straight opposite, taking comfort in loneliness. So there I was, and because he never, throughout our lives together, ever caught me doing anything of the sort, he had gotten this image of me that I didn't do stuff like that. That I was "too pure" or something like that, so without making a big scene or anything as we have major respect for each other, he just jokingly said "ahh... so you ARE human...". But as a natural defence, like your mother, I got frustrated and angry, and told him "...not cool bro... not cool..."
The worst thing I got caught doing as a child/teen...? Nah... I'll get banned for sure xD