I have literally no idea what this means and the disk looks good. Have run disk utility and the disk structure looks good. Also gettting strange lock ups on this iMac (late 2009) running Mac OS X 10.9.4. Strange. But serveral a second read:

kernel: cksum: out of data by 20

Looking at the manual page for cksum doesn't help super much, but search for the like "out of data by" gets me to a narkive and they are talking a bout the freebsd kernel and the internet checkusm routine that looks at IP headers and it generates the exact error message whcih happens because the cksum routine gets a memory buffer and a length of the buffer and a skip size and it is not the right lengths. So something is wrong with the network stack on this machine. Could be related to virtualbox which does mess with the stack? Otherwise I need to rebuild the iMac. Sigh.

if (len)
printf("cksum: out of data by %d\n", len);

I did a virtualbox uninstall and a couple of kext did come out, so rebooting now to see if it helped.
I also see the exact error from someone who got this after the OS X 10.8.1 update:

I keep getting these errors in the system log:

kernel[0]: cksum: out of data by 20

From Errirs

I uninstalled virtualbox and this continued. But magically after a few hours, this doesn't appear in the console anymore. STrangE!