interceptor onException not being thrown/called

The code in the interceptor is not anything special it is just this

public void function onException(event, interceptData) {

writeDump(arguments); abort;


Now I can see in the application logs that an invalidEvent is being logged, but the application seems to be running in an endless loop somewhere. In other words the browser has not returned and is still waiting for the server to return something.

Anyone know why this is the case?