using a knife after buttering bread then putting said knife into jar of jam/marmite or other spread and leaving traces of butter from the said knife in the jar...........oh and while I am at it, buttering toast with a knife and going back to the butter with toast crumbs on it to get more butter and leaving toast crumbs in the butter........
only thing worse than that ^^^ is when you find crumbs in the butter or butter in the jam when you staying at someone else's house