Outside of the commonly listed mistakes, I think one of the biggest mistakes a bigger can make is practicing refinement techniques when the should be performing structural work.
I sort of fell into this mistake myself even when I was trying to avoid it. Just this year, I took a hard look at my trees and realized quite of few of them still needed major structural work to build the correct taper and branch thicknesses.
What I mostly mean by this mistake is leaving "as recieved" branches (nursery stock, collect, etc.) and removing the smaller twigs or adventurous buds. This leaves branches that are either too thick or will be too thick in the near future in accordance with the main trunk. Making for an awkward and young looking tree.
By removing the larger branches, you instead focus on the smaller and newer growth to begin building your branches for ramification/division close to the trunk and the thickness years down the line will look more appropriate.
Another aspect of this mistake is working on secondary/tertiary division when the branch is way too long and/or thick for the eventual composition.
The easiest summery of the mistake is essentially, pruning and working in the tree with a short-term outlook (as in thinking and planning for the next year or two) instead of working of a tree with a long-term mindset. Making a tree "good-ish" now instead of amazing years down the road.
I linked some videos that show case some of the ideas I mentioned as well as
@leatherback talking about this sort of error. If he jumps in here, he can probably speak more on the points he talked about.
Part One of Eric's video on a beginner mistake
Part Two showing a follow up on what the trees looked like later on.
Leatherback's Video coming across this mistake
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