My top six candidates:
6) That if you lay sufficient drainage pipe you can build a playable golf course in any old swamp or rice paddy.
5) That what a fairway bunker looks like is important.
4) That golf is a game played by rolling the ball along the ground as opposed to flying it through the air. Far and Sure (tm) is the essence of the game, baby!
3) That a golf course today can somehow reflect the intentions of an architect who has been dead for half a century (or more).
2) That architecture matters more than conditioning to the golfer on the day he is actually playing golf.
And the most important myth of all
1) That a course lacking firm, sandy soil and/or fine, cool-weather grasses and/or frequent wind can make up for one or more of those things by superior architecture. A fine dream, but something that just ain't gonna happen.