By default, entities face south. If you teleport them to face another direction after being summoned, some entities (like the Zombie) will not rotate immediately but rather take 0-3 ticks to complete the rotation (the less you rotate them the shorter. Attempting to rotate an entity 180 degrees will result in the maximum delay of 3 ticks).

However, if you attempt to teleport that entity while it is still rotating, some entities with the NoAI tag (such as zombies) will stop rotating and will be fixed semi-rotated until you stop teleporting them. (Can be fixed by reloading the world like bug MC-103800. This includes zombies, villagers, and other entities that have rotation animation.

Entities without AI, such as armorstands, will be affected by the bug as well, but will not have the rotation animation and instead be visually stuck mid-rotation. E.g. if you summon an armorstand, teleport it to face North and start teleporting it via ^ ^ ^k, it will travel North while it's body will be facing somewhere west or east (it seems at random)

Steps to reproduce:

1) Summon an entity with NoAI (tested for zombies, armor stands)

2) On the same tick teleport it to any location facing any direction (opposite for maximum effect)

3) On the same tick (or up to 3 ticks after it's been summoned) start teleporting it like ^ ^ ^0.1 every tick

4) Once you stop teleporting it, entities with AI will continue rotating until their bodies have completed the rotation, but entities without AI will be stuck with their heads and their bodies mid-rotation (heads will be rotated more in the right direction, but still limited to 90 degrees more than their bodies).

To avoid this, start repeatedly teleporting the entity after the third tick.