apt line - date sting parsable by date(1)

 deb http://snapshot.debian.net/archive/date/datestr/debian unstable main contrib non-free
 deb http://snapshot.debian.net/archive/date/datestr/debian-non-US unstable/non-US main contrib
 deb-src http://snapshot.debian.net/archive/date/datestr/debian unstable main contrib non-free
 deb-src http://snapshot.debian.net/archive/date/datestr/debian-non-US unstable/non-US main contrib non-free

It can be used to apt-get delayed unstable, so you can avoid critical bugs, watching BTS reports :-)
If a package is found to be very dangerous (many critical/grave/serious bugs submitted for the same issues), you can hold the package not to upgrade to dangerous version.