sudo apt-get update error: E: Splitting of file /var/lib/apt/lists/si.archive.ubuntu.com

You can use

sudo rm /var/lib/apt/lists/*

in order to remove all the lists and

sudo apt-get update

to download the lists again

This just happened on my system too, and the reason was my /tmp disk was full.

After removing some files from /tmp, apt-get update started working again.

I also added a bug report at Launchpad