Cascading effects that result in global conflict.
The first year will herald a glorious and wonderful year for those parts of the world who believe in Santa and somehow still utilize written letters to him.
As those who actually followed the custom start reporting the arrival of gifts, the world starts taking notice that perhaps there is more than we knew.
Religious leaders across the world start reacting.
Some declare this to be the proof that the Christian belief is correct.
Other already extremist movements deny the truth, portraying the events as some kind of sinister plot.
The divide between already opposed religions grows wider, the world has become a tiny bit more fractured and a tiny bit worse.
The one time occurrence causes many to try and exploit the system.
People start preparing for the next year.
Creating ridiculous lists of wishes, both material and immaterial.
Opposing movements declare that the resurgence of <PERSON> was because of insert odd reason here.
They believe that those who are attempting to exploit the system will cause its end.
Or worse, cause divine retaliation.
The first acts of violence occur as devout <PERSON>-ists try and prevent children from creating wish-lists.
The next year's Santa season only makes things worse.
As people start manipulating children, riches and destruction are asked for in vast quantities.
While one man asked for a fancy car, his neighbor asked for the tools to destroy that very car.
Countries around the world start demanding a ban on Christmas, to prevent the chaos.
Christian nations see this as an attack on their faith.
Before long, it becomes clear that there is no limit to the possible gifts bestowed by <PERSON>.
And as chaotic as the world is, a world with unlimited resources is vastly worse.
In a misguided attempt to protect themselves, several nations around the world declare war on each other, and the third world war begins.
Thanks <PERSON>!
Other random effects:
One-child policies abandoned.
The end of pine trees across the world as a global harvest ends their lives.
Literacy goes through the roof, as wish lists become more important.
Elf fiction becomes thoroughly odd.
Slavery discussions ensue around the world, with movements to save elves.
The church of Santa becomes one of the largest religions.
Several people are released from insane asylums.
Doll and toy car manufacturers around the world file for bankruptcy.
News broadcasters around the world go on the hunt for fat bearded men.
Fat bearded men around the world go into hiding. 72