Same problem here with UE50F6400.
Black spot appeared a couple of days ago in lower left corner, expands slowly along lower edge, and extremely hot to touch. Slightly melted the plastic, so unplugged it and searched for solution, which led me here. Also vertical coloured bars over the whole screen.
Surprised this fire risk has not been part of a recall.
My turn now
My Samsung TV (50" J6200) nearly burnt my house down. Out of the blue, it suddenly started heating up and display started dying out. Out of curiosity when I touched the sceen, it was red hot. I immediately turned it off. If it has been ON for some more time, my house would be toast. It is a Fire Hazard waiting to happen. Why was this never addressed and recalled?
I had this just occur to my kids today. I have the tv set up in my small children's room, and I notice vertical and horizontal lines along with the corner melting, which was extremely hot. Luckily none of my children decided to try to touch it. I purchased this 6300 series TV only a few years ago, and it's had minimal use for the past 2 years, please assist Samsung!