PIFs
I'm sure we've all got our own stories about these, but for me it was being left in front of the TV when I was about 3 or 4 when one came on about fire safety. I can't remember much about it apart from animated flames, but I was terrified of fire for a good few years after that. Didn't help that the house across the road caught fire about the same time so I got to see it for real.
(mhoulden, Thu 5 Apr 2012, 17:16, 2 replies)

Yes, same.
I watched an episode of 999 where someone's house was on fire. The woman got her kids out, and then kept going back to her aged father-in-law's room, opening the door and talking reassuringly that the fire department would be here soon. Opening the door like that eventually caused a fireball or something, the woman was mutilated and the poor chap died. I had nightmares for about a year afterwards, and also decided that if my house was on fire I'd be ignoring any cries for help from behind closed doors.
(Sivvus trolley collision in the fruit aisle led to a jam, Thu 5 Apr 2012, 18:26, closed)

the spirit of dark water
creepier than a one-eyed eager uncle in sweatpants
( Smash Monkey lowering the tone of the whole internet, Fri 6 Apr 2012, 20:45, closed)