Error handlers' blocks are attached to processes and invoked when an error (such as self error: ...) occurs. This facility is the basis of the exception handling mechanism. As the logical process winds its way across various spaces, it may register error handlers using traditional mechanisms such as the when:do: instance method of Block.

When invocation handlers are configured to forward exceptions (see the forwardExceptions configuration option), error handlers are propagated across spaces with the logical process. The propagated handler is installed as the error handler for the process which evaluates the request. When an exception occurs, the active process' error handler is evaluated. If the handler is remote, a remote message is sent and the error is handled where the error handler was installed. This mechanism transparently implements remote exception handling. Note however that this mechanism only catches exceptions thrown in user code.