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?