When I run an Arquillian test that uses ShrinkWrap descriptors, I get a nonsensical SAXParseException saying that "content is not allowed in prolog". Why?

Most likely you've passed a file name where the descriptors API expected a string containing XML data. It's an easy mistake to make - compare the following incorrect example with the corrected example that follows it:

BeansDescriptor beansXml = Descriptors.importAs(BeansDescriptor.class)
                .from("src/main/webapp/WEB-INF/beans.xml");

vs the corrected:

BeansDescriptor beansXml = Descriptors.importAs(BeansDescriptor.class)
                .from(new File("src/main/webapp/WEB-INF/beans.xml"));

So, next time you see an trace like this from one of your tests, you'll know what's wrong:

java.lang.RuntimeException: Could not invoke deployment method: public static org.jboss.shrinkwrap.api.spec.JavaArchive com.example.shrinkwraptestsk
eleton.DemoTest.createDeployment()
   ....
Caused by: java.lang.reflect.InvocationTargetException
   ....
Caused by: org.jboss.shrinkwrap.descriptor.api.DescriptorImportException: Could not import XML from stream
   ....
Caused by: org.xml.sax.SAXParseException; lineNumber: 1; columnNumber: 1; Content is not allowed in prolog.

There's an open issue with a proposed API change to make this error harder to make.