A more sneaky thing than filling balloons with exploding gas is to fill a glove and leave it where you are welding or brazing. Enough gas will stay in the glove to really make anyone jump who walks up and you touch the glove with the flame while they are looking away. About like a very large firecracker.
Of course, you would have to be working on a KOF bike to keep me from catching heck for an off topic post.
Tom Sanders
Lansing, MI USA