I'm sure its not the funniest thing, but it was pretty funny at the time, and it sticks out in my mind.
Me and my friend were walking out of Tesco's, they have a thing where if you try and take your trolley past a certain point, the trolley locks.
This guy was pushing his trolley full of food, and tried to take it passed the point, the trolley locked and he carried on trying to push it, the trolley fell over and he tripped over it landing flat on his face. He then got up and started kicking the trolley and screaming at it! "smurfing trolley!" It was pretty god damn funny at the time.