I can remember being at a birthday party and having my first helium balloon experience. I was having a whale of a time, sucking out the gas and making everyone laugh with my cool sounding "jockey" voice.
If only I'd known when to stop....
It seemed as though we were running out of balloons so I pounced at the opportunity when I saw one tucked away in a box. Without hesitation, I ripped off the top, put my lips around the end and eagerly took in about a litre of human air. Ugh.
It's still probably the most disgusting experience I've ever had and I can still remember the foul "taste".