Need help on l4_msgtag_has_error

Hello, Can some one tell me when a msg tag is set to the error code i.e., tag = 0x8000 ? I have a parent thread A, which creates a child thread B, and thread C is assigned as B's pager and exception handler. When thread A destroys B via l4_task_unmap, thread C is invoked and the tag returned from ipc_reply_and_wait cannot pass l4_msgtag_has_error check. This is very weird. When B dies, will it generate some sort of exceptions which will cause C to be invoked? If I ask thread C to ignore this IPC message, it seems fine because thread B is anyway destroyed. But I am still wondering if I did something logically wrong or the tag error can be safely ignored, especially in the situation I described above? Thanks. Chen