Some packets are flagged as INVALID by iptables

I configured a proxypass rule in order to redirect requests from the port 443 to a JBoss server running on port 8080. All works fine except some 503 errors in the Apache log, around 10 per day: In the error log: > [error] (111)Connection refused: proxy: HTTP: attempt to connect to 127.0.0.1:8080 (127.0.0.1) failed In the access log, I've a corresponding 503 error. Nothing in my JBoss log. For each error, coincide a packet flagged as invalid in the kernel log. I performed a tcpdump and for each invalid packet, it was a tcp SYN from the proxy to the JBoss server. Why these packets are flagged as invalid ? Is that a serious problem ? Can I accept all invalid packets from 127.0.0.1 to dport 8080 ?