Had something similar happen to me about a month ago. Went to check in for my MEL-PER flight, and was asked "have you already checked in?". Assumed she meant OLCI, so said yes. BP printed, off we go. Noticed my seat had changed from OLCI but I was further forward so didn't question it.
Got to the gate late in the boarding process. BP rejected at the gate, "already boarded". Um, clearly not. Anyway, sent to the service desk, showed ID and was told to "get on the plane, see if anyone's in your seat, and if they are get an FA to sort it out". So I did, and there was someone in my seat. Find 32K, "um, I think you're in my seat". Mr 32K-who-isn't-me looks at his BP that clearly has my name on it, says "no, I think I'm in the right spot". Upon me asking, "You're not jas, are you?", he looks at his BP again, then quickly vacates his seat with a "I better go get that sorted out!"
He got back on about 10 minutes later, and the first officer's announcement apologies for the delay in departure caused by "late arrival of some passengers, and a passenger with the wrong boarding pass".
So yeah, it can happen.
Got to the gate late in the boarding process. BP rejected at the gate, "already boarded". Um, clearly not. Anyway, sent to the service desk, showed ID and was told to "get on the plane, see if anyone's in your seat, and if they are get an FA to sort it out". So I did, and there was someone in my seat. Find 32K, "um, I think you're in my seat". Mr 32K-who-isn't-me looks at his BP that clearly has my name on it, says "no, I think I'm in the right spot". Upon me asking, "You're not jas, are you?", he looks at his BP again, then quickly vacates his seat with a "I better go get that sorted out!"
He got back on about 10 minutes later, and the first officer's announcement apologies for the delay in departure caused by "late arrival of some passengers, and a passenger with the wrong boarding pass".
So yeah, it can happen.