Back in 1967 I had a similar occurrence when flying as a F/O on the B727.
We were scheduled Sydney-Brisbane and at the same time another B727 was scheduled Sydney-Adelaide.
In their wisdom operations swapped aircraft due to future maintenance and informed everyone EXCEPT us.
We only found out the pax and F/A’s were all going to Adelaide when a passenger asked the F/A why the Pacific Ocean was out there on the right side of the aircraft and not the inland of Australia.
A quick anxious visit to the flight deck by said F/A soon told us that we were the only ones who did not know the correct pax destination.
A hasty radio call to Operations soon clarified matters and we turned for Adelaide and the other B727 turned for Brisbane.
It was tea and bikkies in the Airport Managers office for the Flight Ops people and Some very red faces.
Bit of a laugh though!