We've had the same problem as BarCar on and off for a few years now: right house number, wrong road (a nearby road name beginning with the same letter). And not just at Christmas.

The first year was a very nearby road and close enough that each household would sometimes drop off the mail in person. But then it switched to a road about half a mile away and I just wrote on the envelopes "Delivered incorrectly to x address" and put them in a post box. It happened so frequently that the other household sent us a thank you note!

Some of the letters were official correspondence from banks etc, so it wasn't a case them being incorrectly or illegibly addressed. I thought mail was scanned electronically and filtered by post code so I don't understand how post for one road can end up in the pigeon hole (or whatever they use) for another road.

Perhaps it's a Royal Mail initiative to encourage communities to get to know one another?