Git
GitHub Permission denied (publickey)
When git clone, pull or push over SSH fails with this error, GitHub did not accept your SSH key. Usually the key is not generated, not added to your account, or not loaded into the agent.
Git fatal: not a git repository
This error means you ran a git command somewhere git does not recognise as a repository. Usually you are in the wrong directory, or the repo was never initialised.
Git: Updates were rejected (non-fast-forward)
A rejected push means the remote branch has commits you do not have locally. Git refuses to overwrite them. The fix is to integrate the remote changes first, not to force-push.
Git fatal: refusing to merge unrelated histories
This error means Git found no common ancestor between the two branches you are merging. It is a safety check, and there is a one-flag override once you are sure the merge is intentional.
Change casing of file or directory in Git
Renaming already commited files or directories only for casing in Git will be ignored and will not show up as a change you can commit.
Removing tracked files in Git that should have been ignored
Removing files that has been committed earlier in your repository is something you encouter once in a while. Read here how you can do this.