I'm using a Samsung AU7100 43" TV and am watching iPlayer via a Roku. Most of the time the Roku operates in 60hz but it switches to 50hz when using iPlayer.
I have noticed that when it switches from 60hz to 50hz, the Picture Clarity settings appear to turn on, giving the image a weird smoothing effect. This is despite the setting saying it's Off when I look at the menu.
Turning the TV off and back on corrects the problem, but it's very annoying to have to do this every time.