This happened to me, and it was because the file /var/lib/apt/lists/archive.ubuntu.com_ubuntu_dists_precise_main_i18n_Translation-en had been created while I was on a public wifi that redirected all HTTP traffic to their authentication page. I deleted the offending file, ran apt-get update, and everything worked swimmingly.
This happened to me, and it was because the file /var/lib/ apt/lists/ archive. ubuntu. com_ubuntu_ dists_precise_ main_i18n_ Translation- en had been created while I was on a public wifi that redirected all HTTP traffic to their authentication page. I deleted the offending file, ran apt-get update, and everything worked swimmingly.