This happened when I was 15 years old and always makes me laugh. I had a cool mom.
While I played with myself quite a lot as a pre-teen, I didn't have my first orgasm until I was 15 and once I could shoot, I never stopped stroking, lol. Anyway, while I usually did jackin' at night in bed, sometimes during the day I would get the urge too.
I didn't want to get caught by my sister or mom, so I would close my door, and open the closet door next to it. You couldn't enter my room without banging one door into another. Inside my closet was a chest of drawers, which allowed me to jack off standing up and I'd shoot my wet load onto the top, and use a sock to wipe up afterwards.
It was laundry day and my mom was busy rushing around the house. In my horny state in the closet I was stroking, assuming I'd be fine for a quickie jo session. However, my mom opened the door and entered my room so quickly (I now wonder if she had known what I was doing all along) that she slipped in and saw me from behind with my pants down, cock in hand.
She closed the door and I felt her still, standing behind me, 'Sorry honey...it's okay,' she said in a whisper. 'You can finish.'
And with those comforting words I shot a big load of milky white cum, and then another across the top of the chest of drawers. I was out of breath, knees wobbly, head spinning.
As I looked over my shoulder at my mom, I could feel a wave of regret and guilt when it was her that began apologizing. 'I'm sorry, I should've knocked. I will always knock from now on,' she said as she handed me a hand towel (such a mom thing to do!).
I nodded and began pulling up my jeans around my ankles. She then said, 'You're a sexual being just like everybody else in this house. I'm glad you find time for a release. It's healthy and normal. Moms do it too,' she said as she set t-shirts on my bed, opened the door and exited my room.
After that she kept her promise and always knocked before entering my room. On a few occasions I asked her to come back in a few minutes (I was stroking) and after awhile, I let her drop things off in my room even if I was hard or touching myself. It became kind of like an inside joke between us. She'd pause and make a quick comment like, 'Hmmm...looks fun' or 'Use a towel, not the bed sheet' and off she'd go.
What a cool mom...