I know the feeling well.
The worst time was during the 2007 royal london watches grand prix snooker semi-final. My mate worked at the AECC and managed to score me and a couple of mates tickets to see Higgins-Fu. Towards the end of the semi final John had lost some frames to Fu. At one point Fu cocked something up and provided Higgins an opportunity to redeem himself. At this point most of the spectators let out a "Go on John!", all except one person, who happened to be a mong. He let out a "Mnnnnnggg Jomnngh!". I wasn't sure what this noise was at first until i could here the broken breathing of my mate next to me trying his best not to giggle. I turned to see both mates trying to contain themselves, which resulted in me giggling like a loon too. We spent the next half an hour taking it in turns to compose ourselves, only for one of us to start giggling and set each other off. Sat beside me was a seven year old girl and her dad. At one point she asked her dad what we were laughing at, and he let out a rather pissed off "I don't know".
( Slippery Mick ‏, Sun 1 Feb 2009, 23:22, Reply)