Not quite myth, but close. My Italian-threaded fixed cup came loose once, 20 years ago, after I had installed the replacement upgrade myself, must not have got it RFT. Never came loose again. I also have a French-threaded cup that had sat in there obediently for 30 years or so, outliving probably half-a-dozen spindles. Then when I saw a Campy French-threaded BB on eBay last year I loosened it a bit to be sure it really was French before I placed a bid. It was, so I tightened it back up but not RFT. Sure enough, just around the time the new BB arrived in the mail, the old one came loose. It's replacement hasn't budged, and it is in there RFT I tell you.