Under the old cc-by-sa 3.0 license, Stack Exchange's license to show contributions is potentially perpetual, but violating the license, it gets revoked. This follows from the definition of "You":
"You" means an individual or entity exercising rights under this License who has not previously violated the terms of this License with respect to the Work, or who has received express permission from the Licensor to exercise rights under this License despite a previous violation.
and is also covered in the Creative Commons FAQ. Even after an accidental violation that is prompty corrected, explicit permission is needed from the contributor to reinstate the old license.
How does this play with the new license that SE intends to require for new code contributions? What happens when a user takes code from one of the many old cc-by-sa-licensed answers, and posts it in a new answer? That someone else has no rights to relicense it. As a result of that user's action, SE will be distributing it in a way not permitted by the copyright holder (the original contributor). Doesn't this mean that SE too will at that point be in violation of the cc-by-sa license and have its license to the old code revoked?
Am I totally off base here? If not, is this a problem SE intends to fix somehow, or will it just be ignored in hopes that it doesn't come up with any important content?