SAX is the underlying mechanism in Orbeon Forms by which processors receive and generate XML data. Given only the constraints of the SAX API, it is possible for a processor to generate an invalid sequence of SAX events. Another processor that receives that invalid sequence of events may or may not be able to deal with it without throwing an exception. Some processors try to process invalid SAX events, while others throw exceptions. This means that when a processor generating an invalid sequence of SAX events is used in a pipeline, the problem might go unnoticed, or it might cause some other processor downstream to throw an exception.