Back in the days of BBS's and Newsgroups - and before Star Wars Insider (early 90's), there was lots of talk about Star Wars lore and theories of what the other episodes might include if they were ever made. One of the things that was always talked about as fact was that Anakin/Vader and Obi-Wan had dueled on a volcano and the fight resulted in Vader being "more machine than man".
This, of course, ended up being Vader/Obi-Wan fighting on the volcanic world of Mustafar in Episode 3.
I've spoken to a number of people and we can all remember hearing this often repeated over the years, but none of us can remember just where we heard this. It was never mentioned in the movies or novelizations. Nor was it mentioned in any of the Han Solo/Lando Calrissian books which served as sort-of-prequels at the time. For it to have been such a pervasive and universally accepted story, it must have had some somewhat-official origin (an interview, a back of a trading card, something).
Where did this "fact" originate?