One I have seen written a lot and in a few amateur cook videos is to "seal" the meat by searing it. Nope, that has been debunked repeatedly.
I dislike when any recipes say "The best". I don't mind someone calling it "my best" or "my favourite" or similar. But, "The best", hmph. I might know a version that I think is better.
I know that there are other "facts" that I have seen or heard stated in cooking shows/videos/online recipes. I can't think of any others at the moment. I'm sure something will come to me.
I just remembered one. "You should never fry with olive oil." Why? Just because it's good in salad dressing? EVOO is fine to at least 350°F. I use it all the time for frying. It's not a problem. I used to be sparing with the EVOO, but it didn't get used enough and would start to smell wrong. Since I started using it for frying, that never happens.