Xslt

An XPath expression uses a namespace prefix that is not declared in the stylesheet. Declaring the prefix on xsl:stylesheet fixes it.

A type error: an expression returned a sequence or type that does not match what the function, operator or comparison expects. The strict type rules of XPath 2.0+ are usually behind it.

A practical triage guide to XSLT and Saxon errors — XPST0017, XPTY0004, XPDY0002, 'Content is not allowed in prolog' and more: what each means and the fastest fix.

An honest comparison of XSLT and MuleSoft DataWeave for XML (and JSON) transformations: portability, tooling, learning curve, performance — and when each one wins.

Two xsl:result-document instructions resolved to the same output URI — usually a static href inside a loop. Make the href dynamic.

Not a processor bug: an xsl:message with terminate="yes" fired. The stylesheet stopped itself — read the message text to see why.

Two or more templates match the same node with equal priority. Saxon warns (or errors in strict mode) and picks the last one declared — make the intent explicit.

An XSLT element appears somewhere the language grammar does not allow: wrong nesting, wrong order, or content at the stylesheet top level that must live inside a template.

Two xsl:variable or xsl:param declarations with the same name exist at the same level — often caused by a stylesheet being included twice.

An honest comparison of the free online XSLT testers that actually support XSLT 3.0 — Saxon HE vs Saxon-JS, multiple inputs, validation and trace. Which to pick and why.