Macbook Pro Mid-2012 Retina Screen Flicker
Certain MacBook Pros made between February 2011 and December 2013 were found by Apple to be defective.  Apple is offering to repair them for free.  The symptoms were:


  
  Distorted or scrambled video on the computer screen
  No video on the computer screen (or external display) even though the computer is on
  Computer restarts unexpectedly
  


Affected Models:


MacBook Pro (15-inch Early 2011)
MacBook Pro (15-inch, Late 2011)
MacBook Pro (Retina, 15-inch, Mid 2012)
MacBook Pro (17-inch Early 2011)
MacBook Pro (17-inch Late 2011)
MacBook Pro (Retina, 15 inch, Early 2013)


You have two repair options:

Carry In


Bring your MacBook Pro to a participating provider - Apple Retail Store or Apple Authorized Service Provider.
A technician will run a diagnostic test on the MacBook Pro to verify eligibility.
Repair turn around time will vary based upon parts and technician availability.
You will be notified when your MacBook Pro is ready for pickup.


Mail In


Call Apple and request a postage paid box to send your MacBook Pro to the local Apple Repair Center.
A technician will run a diagnostic test on your MacBook Pro to verify eligibility.
The repair process takes approximately 5-7 days from the time your MacBook Pro is received at the repair center until it is returned to you.
After it has been serviced, your MacBook Pro will be returned to you by mail.


See Apple's page on this topic for more info.