Saxon

The best approach is probably to use a schema-aware transformation: validate the source document against a schema before it gets processed by the XSLT.

Another approach is to put extra logic in the stylesheet: before you try to convert a string to xs:decimal (typically with the constructor function xs:decimal(expr)), insert a test that the value is actually castable:

  Value at line {saxon:line-number(XXX))} is not a valid xs:decimal value

making sure that line-numbering of source documents is enabled, e.g. by setting the appropriate property on the DocumentBuilder.

A third approach which may sometimes work is to look at the Context property of the XPathException within the thrown exception. If this is present you can call getContextItem() to find out what was the context item at the point of failure, and if this is a node, you can call getLineNumber() to find its line number. However, this will give a "wrong" answer if the value you were converting wasn't actually the context item.