I fail to see how this is a major security threat. A MAJOR balls-up, yes. Passengers had a valid ticket, even though entered into the wrong flight and bags travelled with passengers.
My comments are not to defend the agent involved, but what i do fail to see is how the passengers managed to get through the whole system without noticing. Its a basic survival rule of travelling, know where you are going and who with. If the passengers were aware of where, when and what flight they were travelling on, this could've been resolved a few thousand miles earlier. The flight numbers AND destination would not have matched the tickets.
If a passenger was travelling from Lanzarote to Cardiff, would the boarding pass state "Lanzarote/Arrecife - Wales" I highly doubt it.
It has happened at Cardiff, with both handling agents. It happens all the time, all over the world. This one just reached the press.