Error with an Apache in front of Tomcat using AJP connector and mod_proxy
Depends a bit what the situation is - a very common cause is harmless; see:
i.e. apache closing out as the client (browser) has closed the TCP connection and that all having propagated through before tomcat has sent its reply down the internal TCP socket.
So I'd up the log level to confirm it - and then scale it down appropriately for production.