Git "error: The branch 'x' is not fully merged"

As Drew Taylor pointed out, branch deletion with -d only considers the current HEAD in determining if the branch is "fully merged". It will complain even if the branch is merged with some other branch. The error message could definitely be clearer in this regard... You can either checkout the merged branch before deleting, or just use git branch -D. The capital -D will override the check entirely.

^{Note Wording changed in response to the commments. Thanks @slekse}
That is not an error, it is a warning. It means the branch you are about to delete contains commits that are not reachable from any of: its upstream branch, or HEAD (currently checked out revision). In other words, when you might lose commits¹.

In practice it means that you probably amended, rebased or filtered commits and they don't seem identical.

Therefore you could avoid the warning by checking out a branch that does contain the commits that you're about un-reference by deleting that other branch.²

You will want to verify that you in fact aren't missing any vital commits:

git log --graph --left-right --cherry-pick --oneline master...experiment

This will give you a list of any nonshared between the branches. In case you are curious, there might be a difference without --cherry-pick and this difference could well be the reason for the warning you get:

--cherry-pick

Omit any commit that introduces the same change as another commit on the "other side" when the set of commits are limited with symmetric difference. For example, if you have two branches, A and B, a usual way to list all commits on only one side of them is with --left-right, like the example above in the description of that option. It however shows the commits that were cherry-picked from the other branch (for example, "3rd on b" may be cherry-picked from branch A). With this option, such pairs of commits are excluded from the output.

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¹ they're really only garbage collected after a while, by default. Also, the git-branch command does not check the revision tree of all branches. The warning is there to avoid obvious mistakes.

² (My preference here is to just force the deletion instead, but you might want to have the extra reassurance).