I have just done a quick google search and this is actually far more common than i could have imagined.
Just in recent years i have found dozens and dozens of articles about people ending up in the wrong destination.
Person travelling from Bristol (UK) ends up in Kaunas (Lithuania) instead of in Barcelona (Spain):
A couple hoping for a holiday in the sun ended up in Lithuania after a flight mix-up.
www.bbc.co.uk
Person travelling from Bristol (UK) ends up in Majorca (Spain) instead of in Menorca (Spain):
The pair claim they had their boarding passes verified five times but still ended up on the incorrect aircraft
www.bristolpost.co.uk
Person travelling from Copenhagen (Denmark) ends up in Bologna (Italy) instead of in Berlin (Germany):
It remains unclear how the gate check-in staff allowed the woman to board the Ryanair plane
www.independent.co.uk
Person travelling from Glasgow (UK) ends up in Bristol (UK) instead of in Faro (Portugal):
The passenger was due to travel from Scotland to Portugal but ended up in an English city more than 300 miles from their starting location.
www.businessinsider.com
Person travelling from London Stansted (UK) ends up in Zakynthos (Greece) instead of in Seville (Spain):
The couple were allowed onto the flight despite having boarding passes for a completely different destination
www.manchestereveningnews.co.uk
Person travelling from Manchester (UK) ends up in Paris (France) instead of in Copenhagen (Denmark):
A COUPLE who had booked a Ryanair flight to Denmark were left stunned after they ended up nearly 800 miles away in France just days before Christmas. Simon Forster and Emma Schofield had planned a …
www.thesun.co.uk
Person travelling from Marseille (France) ends up in Madrid (Spain) instead of in London Stansted (UK):
‘We realised all the flight attendants were speaking Spanish,’ says couple of penny dropping
www.independent.co.uk
An entire plane full of passengers travelling from Sundsvall (Sweden) end up in Lulea (Sweden) instead of in Gothenburg (Sweden) after they are boarded on to the wrong flight and the crew did not realise:
Here's how Swedish regional airline Nextjet accidentally operated a flight in the exact opposite direction of passengers thought they were going.
onemileatatime.com
This one is slightly different as it is the fault of the passenger who had booked flights to Bucharest (Romania) instead of Budapest (Hungary):
Talk about last-minute travel plan changes!
www.thrillist.com
Not really an issue but a slightly different one here of someone boarding the wrong flight from Manchester (UK) to Alicante (Spain) when two flights to the same place were departing at the same time:
They blamed the airline's 'lack of organisation' for the mishap.
metro.co.uk
Simon Robinson and his wife Michelle were allowed through the boarding gate and onto the wrong plane
www.cheshire-live.co.uk
So passengers accidentally boarding the wrong flight seems to be far more common than i thought. It seems like half of these cases are either people who had booked disability assistance and been put on the wrong flight or people who arrived late and were rushed through. But i am surprised at just how many of these incidents have happened.