The ability to hold two mutually contradictory ideas in mind simultaneously. And believe both of them. A key skill for a Party member in GeorgeOrwell's NineteenEightyFour.

Wave vs Particle ''So are physicists trying to brainwash us?'' - No, but in this case physicists ''are'' carrying some difficult classical baggage to a world where it doesn't apply.  See QuantumPhysics.

Free will vs determinism  ''A fundamental philosophical debate. You can argue it without being a Party member.''

War is Peace 

Freedom is Slavery

BuyNowAndSave.  (??? huh ??? see below) (YAGNI)

''Why is this contradictory? If I need the thing (that is, there is no option not to buy at all), and the price is going to go up, I should buy now.''

That's the big question--''if'' you have no option not to buy it, there wouldn't be as much incentive for them to make such an offer (other than 'helping' you choose ''where'' and ''when'' to buy it).  But in general, this advertising fnord also attracts people who don't really need an item (and certainly don't need it now).  "Hey, if I buy three, I'll save even more!"

See also "borrowing your way out of debt".

''Ya seriously, anyone with experience in software development has seen this principal applied numerous times, and knows that OpportunityCost can be a huge thing.''

''I once was in a meeting with someone who argued both sides of the case and was angry when he lost the decision. -- JohnFletcher''

''Judge: Mr. Jones, you cannot object to your own client.''
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I suppose this is distinguished from TriteSayingsComeInPairs in that DoubleThink isn't about finding truth - it's about believing truth depending on context.
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See also WhatIsIntegrity