Items appear frozen in pipes. You don't see items moving inside the pipes, but it renders a frozen image of items when they (invisibly) move from one pipe to the next. The result is the pipes appear to hold a rendered of each item that has traversed that pipe. Essentially, a shitload of items are rendered, motionless.

The item does traverse the pipe, but you don't see it move.

ex1

ex2

Is this SMP? Optifine?

I cannot duplicate it in SSP.

I certainly can using the latest version of BuildCraft from the repo, no other mods, in SSP.

SSP and SMP. No Optifiine.

Eh, it worked yesterday. Looks like my tick changes last night broke it.

But @darkager this is NOT 4.0.1.