Why are there extra pieces in my set? I bought an advent calendar, with a minifig or part of a scene behind each door.  Each piece is sealed in a plastic bag and needs to be assembled.  Almost every bag has an extra piece in it.  Is this a mistake when the pieces are being wrapped, a little bonus (yay, 2 crowbars!) or sloppy counting?  

(It's extremely Christmassy - a police station, burglars with a crowbar and axe, police officers with handcuffs - oh, and a tree with presents.)
It isn't a mistake.

Lego weighs the bricks in each bag to ensure that the right components are present. However, it is difficult to weigh the smallest pieces accurately. (This is also why small bits tend to share one bag). If Lego tried to get the component count spot-on there is the danger that they would miss the very rare occasions when one of these small pieces was left out of a box. It would be bad for Lego if there were pieces missing - it would both damage their reputation and lead to expensive support calls. So rather than risk that they intentionally include more of the smallest pieces. That way, if the extra one is missing, nobody minds too much. At the same time, Lego knows that a couple of extra bits will make customers happy, especially as the small pieces are prone to getting lost.

So, you're right to consider the extra pieces as a little bonus.