Index: i386/i386/in_cksum.c
===================================================================
--- i386/i386/in_cksum.c	(revision 241328)
+++ i386/i386/in_cksum.c	(working copy)
@@ -36,6 +36,7 @@
 #include <sys/param.h>
 #include <sys/systm.h>
 #include <sys/mbuf.h>
+#include <sys/kdb.h>
 
 #include <netinet/in.h>
 #include <netinet/in_systm.h>
@@ -204,6 +205,7 @@
 	caddr_t addr;
 	union q_util q_util;
 	union l_util l_util;
+	static int once = 0;
 
         len -= skip;
         for (; skip && m; m = m->m_next) {
@@ -477,8 +479,12 @@
 			su.c[0] = *(char *)w;
 	}
 
-	if (len)
+	if (len) {
 		printf("%s: out of data by %d\n", __func__, len);
+		if (once == 0)
+			kdb_backtrace();
+		once = 1;
+	}
 	if (mlen == -1) {
 		/* The last mbuf has odd # of bytes. Follow the
 		   standard (the odd byte is shifted left by 8 bits) */