I brought a helium balloon on a plane once (well, actually, my then 2-year-old son did) - not a good idea. The balloon popped in the overhead well before we hit our cruising altitude and the noise was much, much louder than any popping balloon I've ever heard. Those sitting around us had been looking seriously into their laptops, but jumped, got pretty frightened and thought for certain a bomb had exploded. We weren't too popular on that flight after I explained the real source of the "explosion."