The smartest folks ask first, something along the lines of, "Can I mix Debian Stable with <something else>?"
Answer: This is a very risky thing to attempt, and is generally regarded as a bad idea for all but the most advanced Debian users.
Debian Wiki wrote:Backporting (recompiling and repackaging) is the only safe way to install packages from Debian Sid or Debian Testing on a Debian stable system. Do not install such packages without backporting. Attempting to "mix" releases, especially by updating your sources.list file, is a sure way to break your system. Recovery in these cases usually involves restore from backup.
(Emphasis added)
Freedom-hating naysayer(s). What could possibly go wrong?
Et tu, StackExchange?? http://unix.stackexchange.com/questions ... ith-debian
But for the benefit of those who are clever enough to learn from others' mistakes, rather than insisting on repeating them:
viewtopic.php?f=5&t=109734
viewtopic.php?f=17&t=111774
viewtopic.php?f=10&t=112395
viewtopic.php?f=6&t=113303
viewtopic.php?f=6&t=117551
Update(s):
Another one bites the dust: viewtopic.php?f=6&t=114861
And another: viewtopic.php?f=30&t=115107
Here's an oldie but a goodie: viewtopic.php?f=10&t=80521
And yet another... viewtopic.php?f=17&t=115381 (When will folks learn??)
Here's a creative excuse: "Ooops. I 'forgot' not to mix repos.": viewtopic.php?f=6&t=117551
"I installed a package from Ubuntu and it wrecked my system": viewtopic.php?t=118013#p556389
"I have a critical need for package X, but wanted to run Stable": viewtopic.php?f=6&t=118829
"If only there were information warning people not to do this!!": viewtopic.php?f=10&t=120632

A personal favorite: "I'm not smart enough to learn from the mistakes of others. Or my own mistakes, for that matter": viewtopic.php?t=123786
"Everything worked fine--until it didn't": viewtopic.php?f=6&t=123878&start=15#p587113
"Wheezy installed TAILS on my system" (Bad Wheezy, no biscuit): viewtopic.php?f=17&t=127706
"Maybe if I don't admit to mixing repos, no one will be able to figure it out": viewtopic.php?t=123923 and also viewtopic.php?f=30&t=125113 and ALSO viewtopic.php?f=10&t=125122 and ALSO viewtopic.php?f=10&t=125149 not to mention viewtopic.php?f=6&t=125856 plus viewtopic.php?f=30&t=126017 AND viewtopic.php?f=7&t=126352 (this last one even includes "what gave it away?"

Here's a novel approach: bork your system twice in two weeks, all the while swearing up and down you've done nothing to it.

Most. Epic. Fail. Ever.: "My mix of six different releases (plus two derivative distros) isn't broken. It just keeps mutating!" viewtopic.php?f=5&t=124248

(I may add more as time and happenstance permit. )
And for the curious and intellectually motivated, a decent enough overview of the technical reason why. You may not like it, but that's the way it is.
Addendum: This may also prove helpful: viewtopic.php?f=30&t=115107&p=543317#p543317