There is not a week goes by where i don't hear the mention of passengers leaving their brains at home when going on holidays but even the dumbest of passengers should have picked this up.
I really can't see that at check-in boarding and info calls to gate that there was no mention of the destination that they were travelling too.
I agree with you Cardiff that this is not a major security threat coming to think of it but i would call it a security threat as a passenger from one flight has got onto the wrong aircraft (no fault of boarding staff). If it can happen by mistake to a genuine pax, i wouldn't like to think what somone with bad intent could do.
It has happened before and i'm sure will happen again.