Adding items to a K2 item (article) works fine. Managing the tags works fine. However, if you add a tag to an article and accidentally click the wrong one, it is impossible to remove it. I see there is an "X" next to each tag, but clicking it does nothing. Is there another way?

I would try it in myphpAdmin.

Yes, I could edit the DB. However, I'm looking for a fix (or known bug) to the GUI issue. I can't expect the client to learn database administration because they can't spell. :)

OK, I guess this K2 component isn't being actively developed anymore. Here is the correct fix for those needing it:

File = administrator/components/com_k2/views/item/tmpl/default.php
At line 108, add new attribute onclick="$K2(this).parent().remove();" to the span.

Just tested it with Joomla! 2.5.14 and 3.1.5. Make sure you are using K2's and Joomla!'s latest version.