BuildCraft version: 7.99.15
Forge version: 14.23.1.2604
Singleplayer or multiplayer: Both
Steps to reproduce:
Make a long line of transport pipes in a superflat then pipe a whole load of items through the pipeline and fly far away from the pipes in the opposite way the blocks is going.

Some of blocks will spill out ontop of the pipes instead of travelling through the pipes to the end of the pipeline when returning to the pipeline