This indicates that the component that is being mapped (not that the OpenSEF is broken). Currently, the components that cause this error message to be prompted are the phpBB and Remository. The error is caused by the style that the components use – they have hard codded the Joomla's default SEF and it conflicts with the OpenSEF.
Hopefully, upgrading these components to the latest version solve the problem and makes mapping aliases for them possible.