I know why there are extra pieces in the sets, but I'm curious to find out which pieces will typically have an extra.
I know that usually 1x1 plates (both square and round) will have an extra included in the set, as well as cheese wedges. But I was surprised to find an extra double cheese wedge included once (I don't remember which set it was, but no, it wasn't a piece I overlooked in the building instructions). I have other sets with double cheese wedges that didn't have an extra, so I wonder:
Is there a list of pieces that will usually come with an extra?
Are there other criteria used to determine whether a piece will have an extra?