That inevitable bit of water that collects in the indentation on the bottom of an upside-down coffee mug in the dishwasher. Mugpuddles are often contaminated with detergent residues or other unidentifiable bits.
Generally considered to be unpreventable, mugpuddle alleviation requires the use of a dish towel to prevent the re-wetting of the dry dishes in the bottom rack.
Don't jiggle the top rack! The mugpuddles will spill all over the plates!