Just to start off this is not exactly conforming to the rules of this area because it isn't exactly creepy and it isn't exactly fiction. I just don't really know where else to post it, and it is a glitch.
I was playing an unmodded single player 1.11 creative world, which I remember specifically because I was messing around with Evokers in a desert village. I was already noticing minor glitches, most of which I attributed to modded blocks not being present (shadows where they were in my modded login was the main one.)
Then off to one side I noticed an odd block. It was light blue but not the same color as cyan or light blue wool/glass. On at least one side (all 6 if I remember correctly but I'm not entirely sure) it had a spiral that was slightly darker blue. There were several of these fairly close together, one group in the village and one in a nearby ravine that I had been building in. Remember that while I was using mods, I was using them in 1.10, not in 1.11.
I started breaking the blocks, but not all of them as I did not wish to lose them. I focused on the ones in the ravine, destroying them all even though there seemed to be more of them than at first. Then I returned to the main village area, where the other patch of them had been significantly smaller.
To my surprise, the blocks were everywhere. They hadn't completely destroyed the village or anything, but the change was noticeable. They were just appearing, one every few seconds, without any obvious identity or reason for being there. No entity was placing them, not that I think "It wAs HErObriNE!! OMg!!!," and breaking them didn't seem to slow them down. They did seem focused on one area, though, so I decided to start using TNT. None of them resisted the explosions and they did not return after all being destroyed. Those that the TNT destroyed dropped light blue shulker boxes (which the original blocks were too light colored and intricate to be mistaken for) which contained nothing. The glitch never recurred, ever. I've been looking around to see if anyone else had the problem, but no one did that I know of.
For reference, the mods I was using at the time were:
-The Roots mod, but a version no longer available because the mod got a total overhaul
-A Ninja Gear mod I wasn't using in that world
-Lucky Block Mod, the basic one
-Emerald & Obsidian mod
And
-Mythical Minecraft, with ores and weapons but no additional mobs.
My question is: does anyone know WHAT THE EXPLETIVE HAPPENED!? It was never a problem, really, and never happened again, but it was MASSIVELY strange and I'd really particularly like to know if anyone else ever had this glitch.