This is a merchant get-out - I had Harrods charge me Ł50 in HKG and then refund me Ł50 in HKD while trying to fix it DCC at Harrods - unable to void transaction in home currency

Don't accept. Chargeback once merchant confirms void not possible.

I know that's not what you want to say, but a void button is not mandatory on Visa terminals.

I found that restaurants and taxis (combining data points from trips to Vancouver in June 2017, November 2017, and most recently September 2018) don't like contactless payments. I've been told the reason is because a tap will bypass the opportunity to enter a gratuity, which did happen one time in a taxi during the June 2017 trip, but I haven't found this to be universal. However, on this trip, every offer to pay via tap at a sit down restaurant (and the above taxi ride) was rejected. I saw no DCC in Victoria in June 2018 with Mrs. Majuki, but we didn't have too many non-tap card transactions (only the Empress Hotel, Butchart Gardens, and Hertz) to compare.

Very odd. Places enter in Calgary will generally tell you to follow the prompts then tap your card (I genuinely can't remember the last time I inserted my card) .

If you try to pay Ł50 but the merchant wants $550 (approximately Ł4/$40 too much), what I meant was that you could offer the merchant to solve this by giving you back Ł5 in cash and then skip the chargeback.

If the merchant charges Ł50 in HKD, refunds Ł50 in HKD and then charges Ł50 in GBP, then an extra Ł50 is locked on your card until the refund is posted to your card. This could be a problem if it happens a lot and for large amounts.

I think I have been DCC'd on an Advanced Purchase at a Hilton-family property in EU. From my credit card statement, it's not clear if I had indeed been DCC'd or just have a crummy exchange rate. Of course, you have no choice given and the only recourse maybe to dispute the charge, but I sort of anticipated this, and the damage is minimal (likely $6, I won't bother). Lesson learned though, use AMEX if this is an option even for an advanced purchase.

I don't know how many front-line staff are aware what they've done the customer and are empowered to offer cash comp like that.

Doubletree in EU has form Hotels charging in USD instead of stated currency (I also have an example from UK)

More likely to be simple DCC than the more underhanded Multi-Currency Charging

Suggest if within 120 days of charging, you stay in the hotel as booked, then chargeback the wrong currency charging (making it clear you're only disputing the currency conversion alone, you are willing to pay the EUR amount)

Recently I was checking out from the Renaissance in AMS using the app and it asked me if would like to pay in USD instead of EUR... with some blabla about how convinient it is.

Wanted to add some datapoints from my latest trip to Europe, using a mix of chip and sig, contactless, chip and pin.
Hotels were prepaid and not chains, so no chance to see DCC in action at checkout.

London - No DCC encountered, all charges in GBP without asking.

Eurostar to Paris was interesting, prices for onboard refreshments were offered in both GBP and EUR, with an insane markup on EUR (something like 1.5x to 1.75x the GBP price), but no DCC beyond that.

Paris - Only one instance of DCC in a high end fashion brand shop. Choice was offered on the pin pad and respected, but nearly a 4% markup on FX if accepted.

Spain - DCC is rampant as expected. All restaurants and most shops offered it. When I used my Chip and PIN Target Redcard Mastercard, the currency choice was after the PIN prompt. Markup was generally between 3-4% if accepted.
At the restaurants, I always told the waiters to charge in EUR, and the choice was respected. No BS.
Same with the shops, offered on the pin pad and choice respected.
At El Corte Ingles, using Samsung Pay NFC bypassed DCC, but DCC on regular cards was not forced.
Oddly the Barcelona metro and Renfe also offered DCC, first time seeing it in a mass transit system.

Overall, not as bad as I was expecting after reading the threads here.

Yes, that changed sometime in the last few years. It wasn't the case 3-4 years ago.

Please always request a chargeback. $6 isn't a lot, but the merchant is hit with a high chargeback handling fee (paid to the banks/card companies as compensation for the time they need to spend on this). If shops have to pay this fee a lot, then they'll maybe stop using DCC.AMEX has a higher exchange rate markup than most other cards (if paying in foreign currency), so it's better to use some other card.

This is definitely a YMMV thing. In fact, there are lots of reports in this thread about Visa having worse exchange rates than both AmEx and MC. The possible ~0.5-1% difference is still going to be better than anything DCC offers in any case.

Using Amex means suffering cherry picking exchange rates tho https://www.hongkongcard.com/forumSE/show/17994

I have found AmEx, on average, to be no worse than Visa or Mastercard, even if they do obfuscate the exchange rates.

This is completely untrue. The data referred to on that thread is many years old. The assertion that "In practice they can cherry pick worst rate within 5 days of posting date" is, in particular, completely false. See the text in the US T&C for current Amex credit cards, which limits the possible choice to the processing date or the previous business day:

UK T&C is even tighter, limited to a single day: