I'll start. Inspired by something DS said earlier...
Aptamil toddler milk
Picture the scene. It's a perfect July day and you have tickets to the London 2012 Olympics. You park your car on the drive of an enterprising householder near Stanmore station and set off for a day in London. You are smug in the knowledge that a sleeepsuit, cup and carton of ready-made Aptamil toddler milk are waiting in the car for your return. That evening, after a day at the Natural History Museum and watching USA vs France at Wembley, you arrive back at your car and settle the toddler ready to sleep. You hand him the cup of milk. He sips. His big blue eyes widen with betrayal and he wails, 'Mummy, it's untasty!' You snatch the cup from his trembling hands, take a sip and know that you are the worst mother ever. You knew you should have stopped for fresh milk 
TLDR: tastes like mud.