Like others I had been getting hash sum mismatch errors when updating kali. I was also having issues with an OpenVAS install on another machine. With 2 machines having issues I looked at my network and saw that the machines were being proxied by my utm (Sophos UTM). I created both hosts and entered them in the transparent mode skip list thereby bypassing the proxy. Problem solved.
tl;dr
kali might need to bypass your proxy to update