I was playing with entities riding entities riding minecarts and I just made a pig riding a slime with the invisibility effect lasting 1000000 seconds riding a MinecartRideable. When I started pushing it, the whole screen turned blue like the sky color and there was a white number 2 in the bottom center of the screen. It seemed like I was in the pause menu because when I clicked near the center, it brought me to the world list. When I logged back in, I was in the same situation and I clicked and went back to the menu. When I clicked cancel, the entire game froze but didn't crash. When I restarted and entered the world it was gone and I could re-summon the stak and push it without a problem. There are no screenshots because I forgot to take some but it was a solid blue sky color with a white 2 in the bottom center(that lines up with the stack of 2 items I had in my inventory). I don't think it has anything to do with the entities, it just seemed random.