What mistakes do first time contributors to Bitcoin Core often make? What mistakes do first time contributors to Bitcoin Core often make with their first pull request?
The most common mistakes I have seen first time contributors make (and this includes me) is:

Not opening the PR from a topic branch of the forked Bitcoin Core repo and instead opening it from the master branch of the forked Bitcoin Core repo

Not adding the correct label at the start of the title of the PR
e.g. doc: Address typo in file X

Not squashing commits when there is no need for multiple commits

Opening a PR for individual typos without running the linter and finding additional typos
e.g. ./test/lint/lint-spelling.sh

Opening a PR for individual typos but not updating test/lint/lint-spelling.ignore-words.txt to ignore false positives identified from running the linter.