It's a minor and very specific glitch caused by an oversight in the targeting system.
There are 3 elements:
An assassin must agro a mob that can see through hide
The assassin must use hide before the mob lands an attack
The other player must target the mob, which is targeting the assassin (see element 1), and use the "select target's target" key before the mob resets
The glitch makes it appear to the assassin that the other player is targeting him, but that isn't happening. The other player's target stays on the mob even after using the "select target's target" key