The Propane gas is filling the balloon - to ready for lift off.
I Got this one, after most of the other balloons
had already gotten offf the ground. This was my
first time to use my 30D & 17-40L at a balloon
event & I lost about 30 shots, because the lens
was accidently set to Manual focus instead of autofocus.
Must have happened while taking the lens out of my bag.
And, YES this has happened again, but now I check it first!