I'm running Testing and the error I was getting was:
Reading package lists... Done
W: GPG error: http://ftp.us.debian.org testing Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 010908312D230C5F
W: You may want to run apt-get update to correct these problems
I did this to fix the problem:
# wget http://ftp-master.debian.org/ziyi_key_2006.asc -O - | apt-key add -
That link to ziyi_key_2006.asc is at the top of http://ftp-master.debian.org
and the advice was from the debian-user mailing list.