Synaptic update error in Debian Jessie

I installed some updates for Debian Jessie today, and later tried to use apt-get to install a package from Experimental, but got this error message:

E: The value 'xulrunner-21.0' is invalid for APT::Default-Release as such a release is not available in the sources

A web search brought me to a post on the Linux Mint forum, where somebody had a similar problem. The advice given, which led to a solution, was to check the sources.list file.

I had a look in mine and found that as well as my Debian Testing sources, there was a Debian Jessie source (added during an update of Synaptic?).

Removing that line got apt-get working again. Seems to be a bug where apt-get thinks Jessie != Testing. Anybody seeing the same error, check

etc/apt/souces.list