In creative mode I came across 3 floating water source blocks (in the pictures I provided). For no particular reason while riding my horse, I fall into these 3 floating water source blocks and the horse forces me off of him. Now the horse is below the 3 water source blocks, I try to mount him 3 or 4 more times but get kicked off each time. I wait till the horse gets out from under the source blocks, try riding him again and now he doesn't kick me off, but as soon as I go back into the water I get kicked off again. Now I try placing a few blocks, such as the coal block, and orange whool, and as soon as I place the blocks they vanish. I also can not spawn any mobs with the spawn eggs. While I am not able to place blocks I looked at my inventory and noticed that my character was in the riding a horse pose even though I was not riding the horse. So I leave my world and re-enter. Upon doing that I spawn underneath the source blocks even though I was no where near the 3 source blocks when i left the game. I do some more testing, I can place blocks, but then I ride the horse again into the water, I can still place blocks, but after riding the horse into the 3 floating source blocks 2 more times, now I can not place blocks again. I leave the game and come back and again, even though i left the game about 10 blocks away from the water source blocks, when I come back into the world it spawns me under the 3 water source blocks.
Can this bug be duplicated, it can be duplicated pretty easily in my world, but Im not sure how easy it will be for other people to find the bug, so here is the world seed I was using: 9083266455427422496
and the location: x:312 y:60.682 z:-243.37
In order to find the bug yourself, besides being at the cords I gave, you need a tamed horse, you need to be in creative mode, and some kind of block to place down after the horse kicks you off a few times. After 2 or 3 times, you should not be able to place blocks.
And sometime to note is that this is not a bug with the horses, I tamed a different horse and rode it into the ocean, it kicked me off, I rode it again and it kicked me off again, i did this about 5 times and then when I went to place a block I was able to. So clearly it has something to do with the 3 floating water source blocks.
edit also, it was not as if the server stopped working or world tics stopped happening since mobs were still moving and hostile mobs could still attack me.