Missing pieces

Anyone ever bought a lego set new and found it didn't have all the pieces? I picked up the modular petshop brand new a couple of weeks ago and there are 2 pieces that are not in the set. There hasn't been any chance for any of them to be lost or misplaced. I have only bought 3 lego sets in the last few years so I am not sure what the process is if i need to find a replacement. Do lego replace pieces? What is the best way to contact them? Does it happen often that not all pieces are there? If they do how long does it take for them to send them?
Thanks for any help.

contact Lego directly via the website. they usually look after you as much as possible.

Last week I had my first set with missing pieces ever. Luckily i have tonnes of spares to accommodate the missing ones.

Contact LEGO and they will send you out the missing parts.

I have had 2 new unopened sets with missing pieces  i had 2 missing pieces in the Tower Of Ornthanc 1x2 & 1x3 black brick.
The second set was Hogwart's castle (4842) was missing a 1x2 brick.

That's not too bad considering i would own about 80-90 Lego sets