Trivial Description
The implementation of the XSLTC XPath boolean() function in the BasisLibrary.booleanF() method contains an incorrect error message that appears when the method receives an inappropriate argument. The else clause passes "number()" to the runtimeError() method, when it should pass the string "boolean()".
It looks to me like somebody got a bit too overzealous in their copy-paste coding. 
In any event, it makes for a killer red herring in the rare event that that error occurs, and the fix is trivial.
The file is src/org/apache/xalan/xsltc/runtime/BasisLibrary.java.