Documents must pass a minimum syntax check called being 'well-formed'.
Documents must conform to the syntax of XML to be well-formed.
There are additional constraints that are also considered in checking for well-formed documents.
Violating these rules is considered a "fatal error":
"Once a fatal error is detected, however, the processor MUST NOT continue normal processing (i.e., it MUST NOT continue to pass character data and information about the document's logical structure to the application in the normal way)."
Here's a bad document: bad.xml