I was recently on a UA flight from EWR to LAS. The following flight from the gate we boarded at was a UA flight to BOS. To make a long story short, passengers for the Boston flight boarded our plane, which caused confusion when two people had the same seating assignment.

With scannable boarding passes, how can it be that both people for the LAS and BOS flight successfully boarded the plane? Does this happen due to human-error, e.g. ignoring computer warnings when a wrong boarding pass is scanned?

Whenever I get a last-minute upgrade, there is an an audible sound and red-blinking light which cannot be overheard/overseen, to indicate that the agent needs to give me a new boarding pass.

I have seen several airports (YVR and SLC come to mind) where your BP is scanned at the podium, and as you proceed down the jetway there are several sub-gates leading to different aircraft. These are designated e.g. 28A, 28B, 28C, etc. It is a fairly easy thing to enter the wrong gate and wrong aircraft if no one is paying attention.

Once, ~10 years ago in SLC pax were scanned and directed to a certain gate, only to find the aircraft hadn't arrived yet .

leaving san pEDRO SULA, ABOUT 20-25 YEARS ago, our plane to neworleans was called. when we started to board, my duty free was not available. i asked, and was told there it is. at the next gate over. ask where the plane that had my duty free bag was going, and sure enough going to new orleans. some 30-50 passengers got on the other plane, and went to miami instead of new orleans. took them over a day to get back, as the airline required them to go first to san pedro sulA. then on to new orleans

i think that was TACA airlines.

It's rarer than it used to be, but does still happen.

Sometimes a barcode won't scan, in which case the gate agent usually just manually keys in the seat number from the boarding pass. They should also check the flight number or destination, but sometimes don't...

I can only speak for AA as I used to work there so unsure about other airlines.

If the boarding pass is scanned for the wrong plane, a red do not board pops up and says wrong flight, so that shouldn't be possible unless the computer glitched. If the seat was already scanned used, then even if the pax tried typing the seat in manually it was say that the seat is not vacant... again unless of some glitch.

I'd say by far the most 2 likely scenarios would be either confusing gates after scanning BP and going to wrong plane somehow. If not that, another very likely option is gate agent wasn't properly paying attention and somehow allowed someone to get by them without scanning their boarding pass.

Sadly it does happen a bit more than you think will a pax will actually fly to the wrong place somehow, only finding out when they are pretty much already there.

I sometimes hear PA announcements, just before the doors close, along the general lines of "If you don't want to be in Chicago three hours from now, this would be a really good time to deplane*." I've paraphrased this announcement several times in the first class of a semester. Every so often someone gets up sheepishly and leaves.
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*I dislike the made-up airline jargon word "deplane" intensely, but that's what they say.

A few months back when I was flying out of DCA, the ORD flight before us had delayed and they moved my CLE flight to the same gate as the delayed ORD flight. Long story short, middle age guy wearing huge headphones sees movement at the gate and gets in line. Scans his boarding pass and gets halfway down the jetway before agent turns and runs down yelling, "Sir, sir, this is the Cleveland flight, not Chicago! Sir!" Scanner apparently did something to alert agent, but neither of them realized it at first.

I always get a little laugh out of those announcements. Awesome UA Captain once welcomed us aboard, saying "Welcome aboard UA1234. We've shut the main cabin door so if Newark isn't a part of your travel plans, it sure is now."

Also on AA it's common for pilots to say "If you do not wish to travel to Los Angeles or have a another reason to deplane now is your time to do so"

A similarish thing happened to me in FLL once when Spirit accidentally boarded two flights (at two adjacent gates -- one to JFK, and one to San Juan), with the jetway leading to the same plane. It became clear within like a minute that there were way more people on the plane than seats.

When I was in Res for DL, I got a call from a guy who was supposed to be in SJC, but somehow got on the plane to SJO. Making even better he didn't have a visa (not a US citizen) and he had checked a handgun. He had been arrested for no visa and the gun when he arrived in Costa Rica. That was not a fun call

This happens with some regularity with Horizon flights between BOI and PDX. At both airports, there are multiple outside gates accessible from the same exit from the terminal. there are always signs indicating each plane's flight number and destination, but people still manage to get on the wrong plane. Not too long ago, a woman was sitting in my seat. She snarkily informed me that I was wrong about her being in my seat. At least until she was asked if she was flying to BOI. She muttered something about San Jose and scurried off the plane.....

how would that work?

checked luggage should go to SJC even if passenger boarded wrong plane to SJO?

(unless we're talking about buying the wrong plane ticket, and TSA/... didn't catch it when you declared it during checkin?)

Pax got to ATL from AMS. Not entirely sure what happened when he collected his bag at customs and passed it on to recheck. Plus the day before it snowed in AMS and everything was chaos.