I'm not quite sure if this is actually a real-world example, but I often get the feeling that advice for self-improvement, especially regarding social interactions, falls in this area. By this I mean the specific "smile, make eye contact, say your mind" kind of advice, not the vague "just be yourself" kind.

I suspect that smiling and eye contact and the like tend to work socially for people who already are good at socialising, and trying to do them when you're not leads to looking weird (that is, even weirder than you might look if you're not trying).

The reason I'm not sure it's a case is that I'm not sure it never works. It could be that believing the advice works makes you confident enough to pull it off, or maybe it just works even if you fake it.