Why am I seeing an "Invalid XML character" error when importing a topic?

Some XML characters are invalid and cannot be imported into DITA CMS.

Symptom

I get an "Invalid XML character' error when trying to import a topic that has the following text:
When you open a note that has special Unicode characters (such as  or ) in the note accumulator, an error is displayed.  

Problem

There are certain characters that are not valid in XML.

Solution

Escape or remove the character from topic prior to importing it into the DITA CMS.