“Some index files failed to download.”  That’s the error I got upon running apt-get update, after having Ubuntu Mate notify me that some of my updates are, er, out of date.  The indexes that failed to download were related to Flux, which I really liked having on my Windows machines.  Turns out that, as of this writing, the repository of Vivid Vervet (Ubuntu 15) for Flux ain’t there.  Take a look for yourself and see.  I suppose I could install an older version, but since I wasn’t interested in getting into any compatibility issues I decided to bail on Flux for now.

But I couldn’t uninstall Flux.  It isn’t installed.  I need to get rid of the repository information.  You’d think that would be as simple as remove-apt-repository blah blah, since adding it is a simple add-apt-repository, but nope.  There’s no remove-apt-repository.  Ultimately I opened the path /etc/apt/sources.list.d as admin, and removed the associated list and save files.  Reran apt-get update and voila – both the errors and the nag referring to out of date updates disappeared.