When I was there in June, the clerk asked Euros or US$. I requested Euros.

She told me that she is often asked by customers about the practical difference between using the two currencies. I explained the conversion fees assessed when the transactions are processed in US$. She was surprised at the cost involved in the conversion and told me that in the future, if asked, she'll know which to recommend.

Ahahaha been there, but used a Unionpay (I can't remember why)

I was there for the first time in 2008 but used cash. This was during the dark times before FlyerTalk when I used a Wells Fargo debit card with a $5 + 3% fee for international ATM use. Fortunately, my frustration with these fees led me to find FlyerTalk. ^

At a Madrid cafe just now, DCC choice appeared after my pin had been authorized and accepted and before the waiter was to make his final entry to close the transaction, rather than my choice earlier on.

Not good. They're trying so hard to make this stuff tricky. The PIN should authorise the final amount...

Did you still have control of the terminal when the DCC prompt appeared? This terminal behavior is disgusting, and is at least one advantage of signature to be able to deface a receipt for a chargeback.

Yes I did have control of the terminal at that time: For Dollars, press 1 or Euros press Green (enter). I had thought I was finished after it appeared to accept my pin, but it was the waiter who clued me in that there was one more step to choose. HOWEVER ...

There was a slip of paper that printed after the pin but I did not receive until after the end of the transaction which I cannot scan here so I will have to dictate with my phone the contents:

Attention! This is not a receipt, please read below

You have the choice of paying your bill in either 12.95 EUROS or USD 14.66
USD 1.00 = 0.8832986 EUROS includes 3% markup over wholesale rate.
Service and exchange rate provided by BBVA on November 6th 2015. NO COMMISSION

Once you have made your choice, you need to tell the person assisting you with this sale which currency you wish to be billed in for this transaction.

( in other words, the waiter held on to that disclaimer paper and just told me to pick one currency)

If DCC is going to exist, I like the fact the logical button press is to refuse.

Indeed. This appears to be a better implementation, and I'm glad the customer retains control of the terminal. This is akin to the quote slip from Taiwan, Thailand, and a few other locations. The process in Taiwan is that the quote slip with the check boxes will print, the customer selects the appropriate box, and the cashier inputs the selection.

It's good at the enter key is the decline option since many people who operate the terminal repeatedly press the enter button.

The social pressure is the kicker. Don't worry you're not alone:

I was told off by my Hongkie mother for wasting her favourite Golf manager's time
Jamar was told off my his potential (also Hongkie) brother-in-law for wasting his time
A girl walked out on Moondog for fighting over a $5 DCC

On the other hand I always think the girl in the latter case isn't FT spouse material - contrast that with my wife sitting for 30 minutes at the St Regis Shenzhen Decanter entrance while I stand over a couple of staff til they void a DCC Visa slip and charge something else.

Hope your GF will be as tolerant of your FT ways as my wife.

That's absolutely true, and I will definitely condone you taking the money.
The reasoning is Chase (as issuer) is the only responsible party to you for the whole Visa settlement system (I've tried writing to Visa Hong Kong's office directly, it's just /dev/null).
Chase has a duty of care to you to charge back per Visa International Operating Regulations. They chose not to cos it is cheaper for them to pay you out of pocket to settle your legal claims against them rather than go down the full chargeback process. They probably might opt the latter if the transaction is huge
I say take it. Even if small, if it happens enough hopefully the issuers are gonna finally gonna do something, like lobby Congress to allow cardholder opt-outs (then promptly opt out everyone).

So a more apt description is "making Peter pay Paul for Paul's non-compliance". But I ask for poetic licence to use the classical proverb...

It's okay. I think the other halves on here are tolerant of our quest to fight DCC as that's what keeps the Krug - or Dom in the case of percysmith ;) - flowing.

However, I am a bit confused by how little resistance DCC typically encounters. Even some regular FlyerTalkers approach the topic with apathy. As I've pointed out in previous posts, this same apathy wouldn't be present if being presented in other forms. For instance, if you were to tabulate a bill based on the menu price and then the waiter brought a bill that was 4% higher without explanation, I believe most people on FlyerTalk would fight it. I don't know why DCC receives preferential treatment even when the end user is aware of what is happening.

I thought contactless knowledge was more widespread (i.e. TFL takes contactless for tube journeys?)
As well, London has plenty of foreigners at any given time, so experience with foreign issued CCs should come quickly (though I do take it this depends on what part of the city you're in!)

I'm mainly thinking outside London here.

Knowledge of contactless use is generally good. But knowledge of how to use the terminal isn't. Some are setup to not give receipts for contactless, or give a prompt which cashiers skip through. As a result, most claim their machine won't print a receipt for such transactions, mainly because they don't know (or can't be bothered) to do a reprint.

Fortunately, I believe DCC is not offered on contactless transactions. My main bugbear is making sure they've entered the correct amount.

I was successful with 'What the Pho' in HK as well.

My first DCC "repair"
 

which was actually my mother's card. She had to sign slips for the transaction in dollars, the cancellation or refund of the transaction in dollars, and a third slip for the transaction in Euros. To their credit, it was our fault for answering dollars automatically without thinking; the waitress was very patient on our behalf. I was a bit concerned that her bank would refuse a second charge (the one in Euros) from the same merchant almost immediately thereafter.