It's when they won't remember you doing it right?
Ds's first birthday soon, I planned on getting him a couple of new toys and a special thing for him to keep when he's older. But then I had my first full payday since before I went on mat leave...and I ended up getting him lots more than I originally planned. 
I fully expect him to ignore the lot and play with the packaging
but he can't become spoiled if he has no concept of wanting and getting can he? He's not getting anything else until Christmas!