Obligatory CGP Grey video:
Brian
Portland International Jetport (Maine) is PWM, from Portland Westbrook Municipal…“Portland” and “Municipal” from when it was a city-owned airfield, and “Westbrook” from the original location of its last runway beacon.
And Presque Isle is PQI.
Denver’s Stapleton Airport (DEN) closed in 1995 and was replaced by Denver International Airport which everyone called DIA. However disappointingly they did not change the IATA code to DIA, they kept it as DEN which is all the branding now. They could have however since the code DIA wasn’t put into use until Doha International Airport opened in 2014.
Minor addition of further facts. ORD is located on what was originally a small non-incorporated community called Orchard Place, for all the orchards there. In WWII, an aircraft manufactory and runway was built there, and that was called Orchard Field.
There is one portion of that video that I completely disagree with, though I may have misunderstood since I watched it quickly. He cites the “Y” and “W” historical weather station explanation of how major Canadian airport codes came to start with “Y”, and then seems to dismiss it by claiming that the ICAO 4-letter codes came first, that the first letter is the region and the second is the country, that the ICAO chose “Y” as the country letter for Canada, and the three-letter IATA codes followed from that. It may be that I’m misunderstanding his claim, but this is just flat-out wrong!
First of all the region + country format applies to Europe and probably many other places, but it does not apply to the US or Canada. In those countries, the region is the country, and the other three letters have no predefined ICAO format. Thus, the “region” for any airport in the US is denoted by “K”, and the other three letters are the same as the IATA code. In Canada, the “region” is denoted by “C”, and the second letter is usually “Y” by convention and not in conformance to any ICAO standard.
And, in fact, the second letter doesn’t even need to be “Y”; it’s “Z” for over a dozen small airports and seaplane ports, like ZFN (ICAO: CZFN) for Tulita airport in the Northwest Territories. And for US airports, the second letter can be anything. It has no special meaning at all…
SXM is Sint Maarten, the airport on the Dutch Side. Called The Princess Juliana Airport, it suffered massive damage during Irma. The following year the airport consisted of a series of tents, with cables running along the ground. They continue to rebuild today.
There’s a tiny airport for small planes in an empty field near Grande Case on the French side. No idea its code.
(starting at 13:30)
He later explains that the “CY” in ICAO codes come from ITU radio codes which in turn come from TELEGRAPGH codes (and a story for another time)
ICAO codes is Alaska often start with P – the FAA designations are all over the place for private fields (Holland airpark somewhat near me is 36WI)
Brian
ETA at 10:15 he explicitly mentions US and Canada do not use the second letter for location
My sister travels regularly between YYZ (Toronto), YUL (Montreal) and YYG (Charlottetown, Prince Edward Island) and counts herself lucky that her checked bags haven’t gone astray yet.
And another one that can be confusing is EWR, for Newark NJ, which serves the NY Metro area. I flew there from LAX sometime in the mid-80s when JFK and LGA (La Guardia) were still the dominant area airports and had a hard time getting a LAX employee to understand that I was going to New-ark, not New York JFK. Maybe now that it’s a hub and the 13th busiest airport in the US, it’s more familiar?
Back in the good old days before barcodes and suchlike I flew from SEA (Seattle, duh) to HLN (Helena MT) with a stop in GEG (Spokane WA). My bag, on the other hand, went to HNL (just guess). Kinda wished I’d gone along.
You’d think. But, actually, outside the US, not so much. A surprising amount of bags that should to to LAX make their way to LOS because, obviously, that should be the code for Los Angeles.
That can’t be true anymore. Everything is automated now.
Couldn’t have made much difference – it’s exactly the same letters, just two of them transposed! 
I hope your luggage came back with a pineapple and a ukulele!
Tangentially related, here we have the FAA approach chart to Portsmouth International in NH (PSM). The image of Tweety bird was added – it’s not on the official chart. Reading the 5-character waypoint names starting at the top left, then moving right, then down, you pass through ITAWT ITAWA PUDYE TTATT. On a missed approach you are directed to climb to 3000 direct to TTATT and on track 165° to IDEED and hold. 
I believe @Richard_Pearse first mentioned this. There are quite a few others, but this is one of the best.
“That can’t be true anymore. Everything is automated now.”
And thank goodness for that.
Nowadays, no luggage ever gets lost! 
Nitpicks from a native Houstonian:
From its founding in the late 1960s, the airport in question with IATA code IAH was named “Houston Intercontinental Airport.” They did later rename it “George Bush Intercontinental Airport” (in honor of George H.W. Bush, not his son), but it has always been “Intercontinental” to local residents.
The other airport in Houston (William P. Hobby Airport, with IATA code HOU) on the south side of the city was known for a time before the opening of IAH as Houston International Airport, but it lost that moniker when it was renamed after Hobby. It also closed for a few years after IAH opened until Southwest Airlines started passenger service out of Hobby in the early 1970s.
Obviously lots of luggage gets lost but not because the wrong airport code was entered. It will match the ticket. I used to travel extensively on business and I learned my lesson. I always pack light and never check my bag.
I know, my comment was tongue in cheek.
I travel a lot too, and have the same philosophy: travel light and never check any luggage.
I’m traveling right now and yesterday I went through EWR … God I hate that airport no matter how the code came about.
EWR = HELL.
At least EWR has restrooms! My feelings about O’Hare are the feelings most other travelers reserve for Newark… It’s a miserable, overgrown airport full of miserable people, and not nearly enough restroom stalls to accommodate them all. Also, the dining options suck, and I’m pretty sure my credit card got skimmed at a kiosk there. When planning a recent business flight, I intentionally picked less than ideal flight times just to avoid a layover at O’Hare.