Same here- too many to choose one ;)
The earliest was at my brother's first Communion- my God parents and his were invited for lunch- very well-to-do all 4 of them. We had gone to buy my first suit and stockings and suspenders- but I hate so much at lunch that I undid the button at the waist (no zip). Mum asked me to go and get something from the kitchen and I stood up- and skirt fell to the ground. Was about 12 at the time- nobody batted an eye-lid- and even my brother didn't joke about it to friends.
Another major boo-boo was during my first visit to OH's family. After lunch, future OH, his brother and mum cleared up the table and went to the kitchen- they would NOT let me help. So I was stuck in lounge with future fil- and there was the Downhill ski Cup on TV. He asked me if I skied- and of course I said that I had done so since I was a toddler. He was puzzled by the sport- so I tried to explain. 'To win a race, the equipment and preparation is as important as the ability of the skier. The one who farts best is often the winner'. He looked bemused and confused- so I expalined further. 'Yes, it is quite an art- a bit like choosing Formula 1 tyres. They have to study the weather forecast, the humidity and temp of the snow at top, middle and bottom- and then fart accordingly'. More surprise and funny looks. Anyhow, in the end, I realised why- and was mortified. Still a family joke 45 years later! Farter in French means to apply wax to the bottom of the skis (lots of different waxes avaialble depending on the type of snow, etc). Hmmm.