>>10187635Generally-speaking you'll get a spare of every 1x1 element that either aren't full-brick height or full 1x1 width. Obviously applies to things like 1x1 tiles and plates, but also clips, sometimes bars, or other minifig accessory/adornment pieces like the carrot stems. I don't know which leaf piece you got a spare of; if it's pic related it could fall under the small-piece criteria, but if you're talking about the larger oldschool foliage pieces it's probably because the treehouse was - I believe - the debut set for those pieces being cast from plant-based plastic and they probably included spares in case they couldn't handle the shipping/retail experience as effectively.
