Minecraft: Why did modded blocks replace each other throughout the world?

Question

I've encountered a rather odd situation in Minecraft, lately.

It first started when I accidentally loaded one of my modded private servers on OptiFine instead of Forge, thus completely wiping all modded objects from the server. When I eventually did swap back to Forge, of course, nothing I placed was there, however, some of the naturally generated modded blocks were replaced by others from completely different mods.

As an example, one of the mods I used, 'Pam's Harvestcraft', naturally generated various types of plants throughout the world hanging from trees and growing out of the ground. When I returned to the devastated world on Forge, these ground plants were replaced with various objects from 'Mr.Crayfish's Furniture Mod', the plants growing out of the ground being replaced by the likes of computers, door bells, fences, etc. and the fruits that would've normally hung from the trees with things like tables and chairs.

What happened here? Though I doubt there is, is there any way to restore the former state of the world, be it either a previous save or simply the original, first-generated world itself?

Answer 1

Loading worlds that were created with a set of different mods as to those you have now almost always leads to some form of corruption; Forge even warns you if you do so. It may be a problem with conflicting item IDs, where the items in Harvestcraft had the same numerical IDs as those in Mr Crayfish's Furniture Mod.

Minecraft doesn't (by default) automatically create backups of your world. You can try loading the world again with the mods you used to create it, however, the corruption is already done, so this likely won't work. As fyrepenguin said, you can type /seed to get the seed and generate the world again, however all your progress will be gone.

In the future, I recommend backing up your worlds and not loading worlds created with different mods.