As a non-native english speaker (french), I was pronouncing those words wrong I guess:
debt, doubt, plumber, subtle, indict, and island
debt, doubt, plumber, subtle, indict, and island
(On the one hand it's a very common word, so someone learning English as a foreign language might have been taught the "correct" pronunciation. On the other hand, it's something that many native speakers of English, particularly younger and less well educated speakers, get "wrong" so teaching the "correct" pronunciation may be assigned a lower priority. Also, for all I know, perhaps the "t" should be pronounced in some dialects of English; I only know standard British English.)
There are many simple words that I only recently learned to pronounce correctly. For instance, "write" is pronounced as "right", and "vegetable" is not pronounced as "table".
In these particular cases, it's not that that I'm unable to generate the proper sounds, but I just never learned the correct pronunciation. Nobody ever corrected me until recently.
Part of the problem there is that English has enough regional accents that we're all exposed to on a day-to-day basis that if you're understandable most people won't realize you're pronouncing something "wrong" instead of "differently".
In true confusing English fashion, "indite" still exists, with a different meaning from "indict".
"Received Pronunciation" (roughly, BBC English) is full of all kinds of historical mispronunciations that have become standard, often as a result of the upper classes adopting some working-class tic as a fashion. Dropping the aitch from "historical" would be an example; it was a cockney habit, adopted by posh people. It's mostly faded among folk that talk proper nowadays (at least in England; aitch-dropping seems to have survived among some groups in the US). But a lot of what's considered standard pronunciation derives from horrible mispronunciations by posh people.
[Edit] "An hotel" (with the aitch sounded) is simply incorrect; You only use "an" if you're going to drop the aitch.
My mother once heard the following sound, brayed loudly in the quad at St. John's College, Oxford: "Eh-oh, heeliaw meekyeh!" (Oh, hello Mickey). That strangled accent was common among the huntin/shootin/fishin crowd.
There was a fellow from the BBC Pronunciation Unit on the radio a couple of weeks ago, explaining (in a jokey way, as if to say it's a fuss about nothing) that "omicron" can be pronounced several ways, and that the unit had chosen OM-ee-cron. I did a term of classical greek at school; the first thing we learned was the alphabet, and we were taught to pronounce it oh-MY-cron. Classicist Mary Beard says it's oh-ME-cron, but I suspect she's inferring the way it might have been pronounced in Ancient Greece.
Boris Johnson is a classicist; I'm sure he was taught to pronounce it the same way I was. But he's using the BBC version, at least in public.
Anyhow, my confidence in the BBC Pronunciation Unit has gone down several notches this winter.
I'll need to check this. If the word came from Old French or any post-Latin Romance language, then the "h" would almost certainly be silent. The "h"s in Vulgar Latin were all silent except among educated speakers.
Pronouncing the "h" in "historical" or "hotel" is almost certainly originally a spelling pronunciation. It may have started out as a hypercorrection, just like pronouncing the "h" in "vehicle" when it should really be "vee-icle".
https://www.grammarphobia.com/blog/2016/02/h-dropping.html
That's an interesting article, and apparently the hhistory of dropped aitches is much more complicated than I suggested. But I don't entirely agree with it. It says it's correct to use the indefinite article before a noun starting with an aspirated aitch; I don't know anybody that talks like that, and it sounds awfully pompous.