Lately, during my apache's error-log, I discovered lots of lines such as this :

[Mon Sep 12 12:34:56 2011] [error] [client ***.***.***.***] 0U, referer: http://www.***.com/ 

"www.***.com" is my domain title. Exactly what does '0U' mean? Responses' code akin to the a few of the error demands are 416. My apache's version is 2.2.14. Can someone let me know? Thanks.