I'm a fairly new jumper. I have 9 jumps, 4 of them freefall. My 9th jump, my friend's 4th jump, we were up in a C-182 at about 1500 feet, and I'm sitting facing rearward behind the pilot's seat. I'm scanning around checking out the wings and the earth below when I see some mist flowing over the wings. Whoever fueled up didn't put the left side gas cap on, and we had to bring it down with 5 jumpers and a pilot. Landing was real smooth. A B-1 bomber pilot was flying.