Mercurial "Not At A Branch Head" Error

Although I write and speak a lot about various kinds of source control (git and subversion are the most popular still as far as I can see), my own development projects are on BitBucket under mercurial (bitbucket also offer git hosting these days, and their tools are great). Recently I was working on an upgrade for BiteStats (note shiny new theme, with thanks to @miss_jwo) and I kept getting this error from hg tag

not at a branch head
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