DISCLAIMER: May affect versions earlier than 1.12-pre7.

Summary: When in gamemode 3 (spectator mode), and you spectate as another player, the particles made from them breaking a block appear to be in a different orientation, even invisible from some angles. I highly doubt this is an intended feature.

Also, the chunk boundaries (shown using F3+G) sometimes move all over the place, even if the player is not moving. The chunk borders also sometimes do not line up and are positioned incorrectly.

How to replicate: Firstly, you'll need to get a friend who owns MC and hop onto an online/LAN server. Then, one of you go into spectator mode and spectate the other player. Turn on your chunk borders (F3+G) while said other player does some stuff (i.e. mine some blocks, move around) and you should see that the particles from the blocks are in a different direction to normal.