I heard one about a mom who took her 3 year old to a play place with a big ball pit. The little boy jumped in the ball pit to play. When he came out he told his mom his "bum bum" hurt. She checked him out in the bathroom, but nothing was amiss. She figured he hit it and she'd find a bruise later. All the way home and all evening he cried about the pain, but she swears nothing was obviously wrong. At 2am he awoke with a raging fever and died when she took him to the ER. Turns out he had the tip of a dirty needle embedded in his cheek. It was in the pit, and he landed on it when he jumped in. He died of heroin overdose.
It's probably not true, and ball pits are all but obsolete now anyway, but pretty scary to a mom none the less.