I tried to change the world (I'm getting a different problem though the same concept) hear it is:
Forge Mod Loader has found world ID mismatches
ID 19407 (ModID: BuildCraft:Transport, type buildcraft.transport.ItemPipe) is missing
ID 19406 (ModID: BuildCraft:Transport, type Buildcraft.transport.ItemPipe) is missing
ID 19408 (ModID: BuildCraft:Transport, type Buildcraft.transport.ItemPipe) is missing
ID 19409 (ModID: BuildCraft:Transport, type Buildcraft.transport.ItemPipe) is missing