The SpongeForge project relies on all mod entities being registered through Forge's EntityRegistry, in order to implement parts of SpongeAPI.

However, it seems that BuildCraft doesn't register EntityMechanicalArm (and possibly more entities - I haven't done an exhaustive search). If BuildCraft could ensure that all of its entities are registered through Forge, we'd greatly appreciate it!

Why would SpongeForge need to ever manipulate EntityMechanicalArm? It's... kind of internal.

@asiekierka: In answer to your first response, it appears to be referenced in TileQuarry, which is referenced server-side through BlockQuarry.

SpongeForge doesn't directly touch any mod entities. However, by its nature of having to implement an entire plugin API, it transforms core Forge classes, such as EntityRegistry (code here.

We transform EntityRegistry in order to expose all registered entities through our API, and generate an EntityType for them. Since EntityMechanicalArm is not registered, it ends up without an EntityType, causing problems without our codebase, as well as any plugins attempting to interact with it.

Oh woops- the arm shouldn't have been added to the world.

No, how did this ever work in vanilla without registering it? Odd.

Oh right, it doesn't save+load atm.

That should fix the issue, whenever pre13 comes out.