Affects Version/s: Minecraft 17w48a, Minecraft 17w49a, Minecraft 17w50a, Minecraft 18w02a, Minecraft 18w06a, Minecraft 18w19b
Fix Version/s: Minecraft 18w21b
/giving yourself a potion with a proper "Potion" tag, but with caps, will crash your game.
gives you a potion with swiftness, but
will immediately crash your game.
This doesn't work for invalid potion id's, like "potato".
Along with that, spawning in a container with the potion in it will crash you the instant you check the contents of the container.
EDIT: Writing any potion ID, valid or invalid, with caps seems to crash your game