A big lesson was simply from many many comments about bass gear from a chap who was 64 at the time been playing since his early teens. He used to rubbish flat wounds, nickel or any one that played anything other than an American fender jazz. He kept telling me that you need to be using certain gear to get "the sound" you need stainless steel strings, and a fender jazz. I kept wondering why he played no gigs, I asked him and found out that he barely practices and kicked out of every band he has ever been in. Lesson l learnt from that is
No matter how good your gear is, how long you have played, how much experience you have, none of that means a damn thing if you are no willing to put the time in to practice and improve your playing.
Same guy enrolled in the same jazz degree as I did, because of his apparent experience he was accepted without audition. Never successfully passed first year even after repeating it three times. For his first recital he asked me to play guitar, he chose the tunes and even though it was his recital I had to teach him how to play because he didn't know how to play them.
Later he got a '66 Fender Jazz which had been converted to fretless and after many comments from other musicians about him playing out of tune, he asked me to intonate it. I said "it's fretless move you finger to where the note is", but I did agree to intonate it. It was only ever so slightly out because he had already had a Luthier look over it when he bought it. I played it, really easy to play in tune, gave it back to him and he was immediately out again and claimed I didn't intonate it properly. I asked him to play it and found he was following the dots that were still there from it's life as a fretted bass. Easy mistake to make for a beginner but for someone who had played for a half a century ***.