The one for my DS when he was 5 - local sports centre, bouncy castle obstacle course thing. All good, kids loved it, parents were milling around drinking tea/coffee and chatting.
I went to set out all the food, plates, balloons, banners etc on my own while DH 'mingled', mainly with his parents who had turned up uninvited (family party was the following day at home, on the actual birthday). The party room was next to the toilets and every few minutes someone's kid came in to use the loo, which was fine, until one boy came in having done a spectacular poo in his pants.
He ran towards the toilet, pulling down his pants on the way and this truly enormous poo plopped out of the back of his trousers. I've never seen anything like it - it was like a tree branch! I was rather shocked, and I half shouted "Oh no! Stop there!" which made him backtrack and tread in it, before he ran off towards the toilet to finish his dastardly deed while trailing poopy footprints everywhere. No parents were in sight, I was all alone, and then he started shouting "can you wipe my bum?"
I was desperately praying his Mum would turn up to find out where he was, or DH would come in to help, but no. Then I discovered he had taken his trousers and pants off, and was just standing there half naked.
I was sweating, tried my best to get him cleaned up with some wet loo roll, put his pants in a bag and binned them, got his trousers back on and sent him on his way, cleaned all the shit off the floor before anyone else trod in it, finished getting all the food and drinks out... it was a nightmare.
MIL came in and said "Oh, this is where you're hiding. Have you not put the balloons up yet?"
Sorry - that was long. But cathartic.