Roy Fisher

After doing some experimenting myself the bug seems to be related to fall distance rather than the lead. If you are in Creative (as of 1.7 Pre-Release) and go up 4 or so blocks and fall down, you create a few particles that barely go a half-block's radius from the landing block's center. However, if one goes up those four blocks then lowers themselves not quite to the ground before going up again and down again for a sizable amount of times – a dozen should suffice – you make a large amount of particles on impact that can spread several blocks; as if you fell from a cliff. It doesn't seem to affect the fall damage you take if you were to switch game modes while in midair, but if you do from real close to the ground the sound for such fall damage still happens; even though you take none.

From this and how the fall damage from leads seems to be related to how long the mob is on said lead that one's falling distance is generally cumulative for as long as they are airborne. This is applying (in most cases) even when the mob stops falling, like how they seem to bounce up and down like a bungee jumper when hanging from a lead. In other words, what MiiNiPaa said half a year ago seems to be right.

It's not exactly affecting much gameplay-wise, but can be annoying at times if you're trying to lift a mob say... out of an exposed ravine and back to the surface. Can be bypassed with a 2+ block deep lake or pool of sorts, though. Naturally, Chickens are exempt from all this; since they don't take any fall damage.

^ The way things are going, he'll learn the same way the impact of the random fire flickering hit the developers; the hard way. I'm surprised I was the only one who brought that up, since it did end up getting an update to squash that specific bug. I hope you guys are ready for another string of updates like 1.4 had for a while, because I'm getting a pretty big sense of deja vu.

For those who don't know yet, this bug is MUCH worse in motion than on stills. The stray pixels change just about every time you move, be it your head (the mouse) or your body (WASD). It's also very prevalent during the daytime in dark caverns, though not so much at night where it's tough to notice without a Potion of Night Vision or maybe another light source. Also, when looking down I can see them being blue or sometimes even red, which leads me to believe that these stray pixels are transparent rather than colored (kinda like the Glowstone "x-ray" trick).

I've only been able to spot it on stone so far, but considering that cave exploring is a major part of Minecraft and Stone is the main block of the underground that alone seems to be a rather annoying bug; one that, if in the official release, will likely cause an even worse ruckus than above. If anyone here remembers the hooplah over the random torch/furnace fire particle bug way back when, you know what I'm talking about; and that one had a work-around unlike here.

@Leo: This was right at the spawn point of a freshly made Classic Superflat world on Creative, so it couldn't be much more than 1000 blocks from (0, 5, 0). Structure Generation was off, so there was no villages or whatnot nearby. Torches only seem to flicker particles to the North and West (meaning their X and Z generation coordinates are randomly smaller than they should be), as seen after testing a single torch for several minutes. Also, when trying this bug from the face-view just in front of the torch in the direction in question, the flicker showed up at around my feet for whatever reason.

The Enchantment Table letters will probably need a separate patching to fix.

It's doing so on the latest Pre-Release as well (regarding book particles). It also comes from flame particles from lit Furnaces and Mob Spawners, so the problem seems to be in the particle generation.

After looking at how it's affecting the letters it seems that the letter of the particle that flashes ends up being the same as next one to start coming out of one of the bookshelves. Furthermore, the flashed particle is much brighter than the one that flows in (under Night Vision, anyway). In other words, it seems that something is causing them to generate in the wrong spot and brightness then correct themselves a tick later. A seemingly cheap way around this would be to make the particles invisible for their first tick around, but that would only cover up the problem rather than actually fix what's making them generate incorrectly in the first place.