Mine is the fact that I didn't follow through with an opportunity I was given...it's not a guitar related one but it is music...
When I was a kid at school I played the drums...I started at around 12 years old...I did pretty good for a couple of years and played in the school band and orchestra...My drum teacher had played with the London Symphony orchestra for 20 years...he was a world class percussionist...he took me through my drum grades and got me to the point where he wanted me to go for an audition with the county youth band...I went to the audition and passed and was offered a place...but i never took up the place...for what ever reason...maybe a confidence thing I don't know...but after all that work I turned the place down...!!!
The worst thing is that my teacher also taught the percussionists from the youth band so I would have had ongoing tuition from a world class teacher and also the band toured Europe and played everywhere...!!! I could have been 13 years old and touring Europe...!!!
I look back today and think what an absolute f***ing idiot I was...!!! I could have gone on to work with professional orchestras...god knows I could have even made it into the London Symphony Orchestra myself...!!!
As much as I love playing the guitar I still look back and kick myself for turning down the best chance I ever had to be a professional musician...!!!