If a project contains supplementary characters (for example, one or more emoji characters in a track name, label text or metadata), when saved (Windows only), the character is converted to a surrogate pair, which is illegal in XML 1.0.

Example:
The attached project contains the tag:

<tag name="TITLE" value="asdf &#x1f3a7; asdf"/>

where &#x1f3a7; is a 5 digit HTML entity (hex) for Unicode character U+1F3A7 (headphone symbol).

When the project is saved (on Windows), Audacity converts this character to the surrogate pair: "&#xd83c;&#xdfa7;"
The saved project will now not open (any OS) and will return an error in the form:

"Error: reference to invalid character number at line ..."



Ref:
https://msdn.microsoft.com/en-us/library/ew7hbz7k(VS.71).aspx
https://en.wikipedia.org/wiki/Valid_characters_in_XML