The exception is correctly handled by the comet-actor's exception-handler, if defined, but the thread processing the comet-request is not cancelled so the AJAX-request hangs until LiftRules.cometProcessingTimeout is reached.

The comet-actor thread should be canceled after the exception-handler is executed.

See the example here illustrating the problem: https://github.com/andreak/on-example-cometexceptiontest