Conventions that work best (when they don't come up)

Let me preface this by saying I have no interest in rehashing old threads on Flannery.

Flannery is the most frequently cited example of a convention that works best when it doesn't occur.

The 2 opening itself is about E.V. neutral with 1 and loses an otherwise valuable call.  The advantages occur when you open 1 and have a better defined 1 responses and spade raises.  And that responder can conceal four spades in order to bid NT or a minor.   Whether or not this is all net positive is mute.  The point is that Flannery's advantages occur in absentia.

July 2013 Bridge World (MSC Problem D) offers this problem:

AKJx AJxx --- AKQJT

The problem D spoiler is that partner will respond 1 to 1 and 3 artificial second negative after 2 - 2; 3.  Moderator Berkowitz quips that the followup to 1 could well have been what you do after 1 - all pass?  (With various passive-aggressive possibilities.)

A good chunk of the panel opens 1, which, while solving some rebid problems, isn't exactly a step towards slam opposite say... a 5-2-4-2 zero count where you wouldn't mind being in a grand.  Not that the 2 openers are going to fare much better with the awful rebid problems.

The problem isn't with opening 2 on this monster.  The problem is that the second negative came at unfortunately high level.  One where opener is DESPERATE to hear a distribution showing reply... and we are backed right up against 3NT.

I've heard many, many players on this forum and elsewhere disparage the use of an immediate double-negative over 2 (usually 2.)  And it isn't a great auction when it comes up, especially when opener has hearts.   And it eats up whatever use you would otherwise set aside for 2.  But the strength of this tool isn't its direct implementation.  Much like Flannery, this tool helps you most when it doesn't occur.  Take the Bridge World problem auction:

2 - 2;

3 - ??

Now transfers would be great here (with 3 being a transfer to NT.)  But even without transfers, just the presence of a natural 3 bid helps enormously.  When responder bids 3 they aren't going to hold a 5-card major outside of a 6-5 that is bidding again.  They really have diamonds.  You can use that information to make an informed decision.

But furthermore, you can use this comfort to plan ahead.  A strong 3-suiter need not risk a 1-of-a-minor opening bid.  The immediate double-negative means your 2 auctions will be slicker, because regardless of whether or not partner responds with the double-negative, their next bid over your 3-of-a-minor will be natural, allowing you to recover a fit!

So what are your favorite tools that work best when they don't occur?