Question

I’ve cloned a large, well-maintained repository. By accident I used the -b flag when checking out a different branch. Git says “Switched to a new branch”. Was the pre-existing branch overwritten, is git’s output misleading, or did I make another mistake?

Answer 1

Was the preexisting branch overwrote,

No.
The default branch has been checked out, and a new local branch has been created from its HEAD.

You can see all the branches with:

git branch -a

Simply switch back to the upstream branch of your choice with

git checkout -b aBranch --track origin/aBranch
# if the local branch was already there
git checkout -B aBranch --track origin/aBranch

Note that if you have work in progress, you need first to go a git stash, as explained in “To git checkout without overwriting data“.