The bug

Starting with Snapshot 13w10a, various small particles will visibly jump to the northwest before following their normal path.

First seen with void particles, has since been observed with the small whitish particles seen in water, the dust particles that rise from mycelium, droplets from the bottom of blocks with either lava or water above them, the larger round bubbles in water, the droplets of rain when they land, and the green particles from using bone meal on a plant.

To view the glitch, simply get to where the particular type of particles would appear and note their movement. Facing either northeast or southwest and either straight down or up (for those particles that appear at the bottom of a block) makes this easier.

Code analysis

Code analysis by tterrag can be found in this comment.