What goes wrong in XML Schema (XSD)

Parse errors · 6

These stop the document being read at all. Nothing downstream sees a file with one of them, because it is not XML.

• Attribute redefinedAttribute href redefined (line 1)The same attribute name appears twice on one element, which XML forbids outright. HTML silently keeps the first; XML refuses the document.
• Invalid element nameStartTag: invalid element name (line 2)An element name breaks XML's naming rules — usually because it starts with a digit, contains a space, or uses punctuation XML reserves.
• Comment not terminatedComment not terminated (line 3)A comment was opened with <!-- and never closed, so the rest of the file is swallowed as comment text and the reported line is the end, not the fault.
• Detected an entity reference loopDetected an entity reference loopTwo declared entities refer to each other in a cycle. The parser stops rather than expand forever — the same guard that blocks a billion-laughs attack.
• Unescaped '<' in an attribute valueUnescaped '<' not allowed in attributes values (line 2)A literal < inside an attribute value. XML forbids it there because the parser cannot tell it from the start of a tag, so it must be written &lt;.
• Double hyphen within commentDouble hyphen within comment: <!-- a (line 2)XML forbids -- inside a comment's body, even when the comment is otherwise closed properly. It is one of the specification's genuinely surprising rules.

Schema errors · 8

These appear only once a schema is applied. A document can be perfectly well-formed and fail every one of them.

• The attribute is required but missingElement 'order': The attribute 'id' is required but missing. (line 2)The schema marks an attribute use="required" and the document does not carry it. Well-formed, and rejected by the first consumer that validates.
• The attribute is not allowedElement 'order', attribute 'ref': The attribute 'ref' is not allowed. (line 2)The document carries an attribute the schema never declared. XSD is closed by default: anything not named is forbidden rather than ignored.
• This element is not expectedElement 'note': This element is not expected. (line 10)An element appeared where the schema's content model does not allow one — an extra child, an unknown child, or the right children in the wrong order.
• Missing child element(s)Element 'order': Missing child element(s). Expected is ( line ). (line 2)The element ended before its content model was satisfied. The schema names what it was still waiting for, which makes this the most actionable schema error.
• Not a valid value of the atomic typeElement 'issued': '31/01/2026' is not a valid value of the atomic type 'xs:date'. (line 4)The element's text does not fit its declared datatype. Dates are the usual culprit, because XSD accepts exactly one format and humans write several.
• The value is not an element of the setElement 'state': [facet 'enumeration'] The value 'pending' is not an element of the set {'open', 'closed'}. (line 5)The value is not one of the permitted options. The message lists the full set, which makes it the one schema error that tells you the answer outright.
• The value is not accepted by the patternElement 'code': [facet 'pattern'] The value 'widget' is not accepted by the pattern '[A-Z]{2}-\d{4}'. (line 7)The value does not match the regular expression the schema constrains it with — the usual guard on SKUs, references, IBANs and postcodes.
• No matching global declaration for the validation rootElement 'invoice': No matching global declaration available for the validation root. (line 2)The validator could not find the root element in the schema at all — nearly always a namespace mismatch rather than a genuinely unknown element.

Back to XML Schema (XSD) — root element, namespaces, constraints and a worked example.