I have about 2 that I can remember distinctly.
The first one when I was young and in elementary. I was in gym and everyone was doing sit-ups. Half of the class would be doing the sit-ups while the other half would be sitting on the feet. Anyways, it was my turn and I remember it building up, but let's face it, you can't hold in a fart doing something like sit-ups. When I let it ripped, it was very loud and echoed the whole gym to where you couldn't tell where it came from. The whole class seemed to stop and it was very awkward, but I don't think anybody knew it was me. Except maybe the fat girl that was sitting on my feet.
The second time was in HS when me and a buddy where in a computer class doing some work during a breaking period. I could feel a fart kind of building up inside of me, and was just going to keep it in, but the teacher got up and left. As soon as I heard the door closed I let it rip as loud as I could. My friend hunched over, and was trying to hold in his laughter. I was like wtf, and was like why are you trying to hold your laughter in (this guy normally is laughing all the time). He pointed at the computer screen, but I knew he was pointing past it. I peeked my head over all of the monitors and on the other side I saw the head of the head cheerleader working on something. It looked like she acted like nothing happend, but I knew there was no way she could have missed that. At first I was like dammit, but I thought since my buddy is one of those fat guys, she probably thought it was him.
Probably one of the craziest farts I've ever heard though goes to my brother when I was probably in 1st or 2nd grade. We were sitting at the kitchen table, and we were about to eat breakfast before we went to school. My brother farted, and it sounded like a tune from the Little Mermaid (the aw part). It lasted like 10-15 seconds. I'll never forget that fart.