I know it is possible to get over 99 Balls.
I have 99x Max-Potions and haven't found one since 30+ Pokéstops. Which may answer my question or may be coincidence. I never recieved less than 3 items from a Pokéstop.
Imagine I keep 99 pieces of berries and each potion (don't mind the bag-limit ;) ) and I spin a Pokéstop.
Do I get only Pokéballs? Do I recieve only the items I can carry? Or will another "stack" of items be opened? Or do I have sheer bad luck with getting Max-Potions?