Here are some bad situations you can get yourself into, and how to get yourself out of them.

I did something terribly wrong, please tell me git has a magic time machine!?!

git reflog
# you will see a list of every thing you've done in git, across all branches!
# each one has an index HEAD@{index}
# find the one before you broke everything
git reset HEAD@{index}
# magic time machine

You can use this to get back stuff you accidentally deleted, or just to remove some stuff you tried that broke the repo, or to recover after a bad merge, or just to go back to a time when things actually worked.

I committed and immediately realized I need to make one small change!

# make your change
git add . # or add individual files
git commit --amend
# follow prompts to change or keep the commit message
# now your last commit contains that change!

This usually happens when you commit, then run tests/linters... and FML, you didn't put a space after the equals sign. You could also make the change as a new commit and then do rebase -i in order to squash them both together, but this is about a million times faster.

I need to change the message on my last commit!

git commit --amend
# follow prompts to change the commit message

Stupid commit message formatting requirements :-P

I accidentally committed something to master that should have been on a brand new branch!

# create a new branch from the current state of master
git branch some-new-branch-name
# remove the commit from the master branch
git reset HEAD~ --hard
git checkout some-new-branch-name
# your commit lives in this branch now :)

Note: this doesn't work if you've already pushed to origin, and if you tried other things first, you might need to git reset HEAD@{number} instead of HEAD~.

I accidentally committed to the wrong branch!

# undo the last commit, but leave the changes available
git reset HEAD~ --soft
git stash
# move to the correct branch
git checkout name-of-the-correct-branch
git stash pop
git add . # or add individual files
git commit -m "your message here"
# now your changes are on the correct branch

Note: You can use cherry-pick for this situation too, so take your pick on whatever one makes the most sense to you!

git checkout name-of-the-correct-branch
# grab the last commit to master
git cherry-pick master
# delete it from master
git checkout master
git reset HEAD~ --hard

I tried to run a diff but nothing happened?!

git diff --staged