At some point, every junior is going to mangle something, and need a senior to sit them down and give them the git reflog talk. It's an inevitability, and should be embraced as a natural part of the evolution of a developer.
Probably just making them aware of it. But I can give a slightly longer spiel:
Commits in git are immutable. They're identified by their hash, so they have to be. What's more they have the hash of the previous commits so the whole chain back to the first commit can't be changed. You can only add new chains.
As a consequence, if your main branch points to commit abc123 and your feature branch points to commit def456 then it doesn't matter if you merge, cherry pick, rebase or dance the fandango, if you point those branches back to those commits, the branches must by necessity look identical to the way they looked before you did anything.
And you can find out where they used to point in the reflog.
To explain, I'll borrow the solution that I first saw in a Fog Creek presentation from ages ago ("DVCS University")...
Essentially, if you have more than one person making changes to the same piece of code, the method for resolving them is:
1) Pull - gets their changes
2) Merge - puts their changes together with yours and you can reorganize them at that point.
Note, this was done with Mercurial which doesn't have the concept of a stage, so Pull feels like it has a slightly different meaning when you look at it that way. One [suggestion][1] for Git if you want to achieve the same effect - getting a Fast-Forward at the end - is to Rebase, then Merge last.
Part of me knew this, I simply forgot and wanted to get out of this particular hole.
Just the explanation of "no matter how bad you've fucked up your repo, as long as you haven't run 'git gc' or waited a few years, all you need to fix it is to find the commit hash from git reflog, git checkout that hash, and then git branch to give you an easily accessible reference to the commit". Followed by some pointers on how to efficiently dig through reflog.
It's pretty freeing to realize that it's basically impossible to lose committed code.
Or made a "fresh clone". I've had colleagues nuke a repo when it gets in to a "bad state", start over with `git clone`, repeat the work that git lost...