.I was around 18, and the GF Helga insisted on going to a place in Adelaide called Magic Mountain. Its all water slides. Grab the towel and costume and on a 37C day we drove to the venue. She brought her mother along. Which was okay as we got on fine. Too much looking around shops later and she finally decides its time to enter the water park. I was by then a bit dizzy with the heat but not unwell enough to sit it out. Big mistake. I got changed, through everything into the bag and left the changeroom. Helga was waiting and her eyes went wide. "What?" I asked. "Your costume" she said "You are wearing your KNICKERS!" I must have nearly feinted. Shocked and stunned I looked and realised I was in my flesh toned waist high lycra briefs not the small shorts I had bagged. TOO LATE! My GF's mother was on the spot in her black one piece. She eyed me approvingly. "Sexy costume!" she commented "Never knew you were a bikini swimwear fan," and walked on. My GF whispered..."She thinks your knickers are swimwear...so stick with it!" So for the rest of the time we were there, I did. It wasn't ideal but to change then would have given myself away. I was so unsure if the mother knew or didnt know. But after 2 hours...she insisted on a photo op with me. The GF took the camera all smiles. And as we were posing she whispered (sexily) "I know what you are wearing is not swimwear! Your loss of memory is my photo albums gain!" "How?" I asked. "You do your washing at my home and I have seen on the line. But this is much better...your genitals are all but exposed!" I was stunned at her frankness. A side of her I never saw (just as I was showing a side of myself she had never seen). Helga snapped away offering no support to get me out of the situation. Later during sex Helga would boast how sexy she found my mistake and her mothers desire for the photo op. And I guess in many ways so did I given there was no way to undo what had been done. I had been careless and paid the price.