64 mods. All of my cave air was replaced with tube coral wall fans. In the End, random ore pockets were replaced with waterlogged polished blackstone walls. Several blocks were also completely mismatched; Soul Campfires became Bells when placed, any End Gateways that were summoned via /setblock became waterlogged purpur stair corners, and brick walls, when placed in different formations, would become every which block other than a brick wall. I suspect that almost every block is similar.
I have absolutely no idea what caused this, and figured I should at least have a laugh at it before resetting my world.
update: the server crashed, here's the log if anyone can figure anything out