That can be filed under the "whack customer service" category...!
The set is produced to run at 120Hz. You may notice ghosting and artifacting when you run higher settings, and some have noticed a "triple ball effect" with certain content of footballs or tennis balls, etc. but some have not had an issue whatsoever.
I spoke with a Samsung customer service representative (their 800 number) who told me never to use the 120Hz motion flow feature on anything other than sports and/or fast movement programming. And even then she insisted that I only use the medium or low settings as it could damage the television if left on continuously. She said never to use the high setting. I find it odd given it was one of their selling points. She mentioned that it cautioned against it in the manual but I haven't found anything that addresses it.
If I've missed an earlier thread on this I apologize, but has anyone else heard of this?