Xslt

A raw ampersand appears in the XML. In XML, & always starts an entity reference — literal ampersands must be written as &, and only 5 named entities exist.

An expression uses a relative path (./foo, foo/bar) in a place where there is no context node — typically inside xsl:function or a stylesheet-level variable.

An XPath expression references a variable that does not exist at that point: a typo, or a variable declared inside a block and used outside its scope.

Saxon cannot resolve a function call: the function name is unknown, the number of arguments is wrong, or the function belongs to a newer XSLT/XPath version than the one you selected.

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.