Rob Allen has another Zend Framework-themed post to his blog today looking at handling exceptions in front controllers a bit more correctly than they're currently treated.
If you have a Zend Framework Front Controller plugin which throws an exception, then the action is still executed and then the error action is then called, so that the displayed output shows two actions rendered, with two layouts also rendered. This is almost certainly not what you want or what you expected.
He points out the more correct process it should follow - dispatch the request and catch the error there before the request continues. The error is then tossed to the error controller for correct handling. He includes the code to do just that, showing how to wrap the routing in a try/catch and push the exception over to the error controller with an "error_handler" plugin created with an exception type of "other".