sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 16126D3A3E5C1192
sudo apt-get clean
sudo apt-get update
I got this error when I installed chef-server on a 1GB server and it was eating up all of the memory on the box, leaving nothing for apt-get to work with. After I removed chef, all was well again. Chef starts up its own copy of nginx, and postresql, so it's a huge memory hog.