This is why you are getting the error message that you are.
git checkout can do one of two things. If you just specify a branch and don't specify any paths then it will switch your current branch to the branch that you specified.
git checkout mybranch # switch to branch 'my branch'
If you supply some paths, then git will checkout those paths either from the index or, if you specify a branch, from a given branch.
git checkout myfile # checkout 'myfile' from index
As you can see, there is a potential ambiguity. What should happen if you had a branch called myfile or a file called mybranch?
The way that git resolves this ambiguity is that it tests the parameter to see whether it matches a branch first, and if not it assumes that the parameter refers to a file. If you had a branch and file with the same name you can force git to treat the parameter as a file with this syntax.
git checkout -- myfile # treat 'myfile' as a file
The -b option, which creates a new branch, is only valid when you are using the branch switching form of checkout and not when you are checking out specified files from the index.
git checkout -b newbranch myfile # Illegal. I can't use `-b` when
# I'm checking out files.
If you try git checkout -b newbranch origin/BRANCH and you get this error it means that origin/BRANCH didn't match the name of any branch that you have so git assumed that you must be referring to a file.
To show what remote branch references you have you can do git branch -r. If you don't have a reference to a branch that you think should exist you may have to perform a git fetch to retrieve it from the remote.
If you supply -b but no branch name to base the new branch off, git defaults to using HEAD, i.e. the commit that your current checked out branch is on.
git checkout -b origin/BRANCH
This creates a new local branch called origin/BRANCH based on your current commit. This is, at best, likely to cause you some confusion and doesn't sound like it's what you want at all.
