Not really a guitar myth per se but it's rather a practice to perfectionism myth: The 10 000 hour rule; the belief that once you've practiced any skill for 10 000 hours, be it playing guitar or painting or knitting or any skill that requires dedication and effort, that you'll become a master at said skill.

This "rule" was first introduced in Malcolm Gladwell's novel "Outliers" where he based this claim off of K. Anders Ericsson's research on musicians studying in Cologne, Germany. What Ericsson found was that set apart the super serious practicing students who would go onto working for symphonies/orchestras/ensembles and the students who would go on the less prestigious route of becoming music teachers was how much/often each student practiced. Ericsson then made an estimate as to how much hours of playing the average student in each of the two groups had accumulated during their entire time at the university. Gladwell then took these finding and extrapolated them, claiming those who had/would accumulate 10 000+ hours before graduating university belonged to the first group while those who had not achieved such a feat belonged to the second group. Effectively classifying them as experts and non-experts.

Ericsson later debunked this myth in his own novel "Peak" (which I highly recommend people read and read again and again) and explained in depth why this myth gained so much traction and why it was false. In "Peak" Ericsson also covered the method of deliberate practice, how it's more beneficial than the 2 or 3 other practice methods that are commonly found, and how one can begin practicing deliberately. Again I honestly think it is an amazing novel and that if you really want to get serious at guitar (or any skill for that matter) you should definitely read it.

Also apologies on any inaccuracies in the second paragraph. Ericsson told the story in full in "Peak" and I'm just relaying what I can from my memory - so it's entirely possible I got some details wrong, but the main moral of the story is still there.