Forge Version: 47.3.6
Mod Version: 1.20.1-1.4.1

Use Spectre Flare near yourself to spectate, left-click (attack) the player in this state, and you'll get kicked from the world for "Connection Lost: Attempting to attack an invalid entity", although this is in single-player.

Additionally, when you enter this world again, the Spectre Flare works wrongly, it is just thrown out and has no effect. This won't go back to normal until the player dies once.

Maybe not letting the player do left/right-click while spectating in Spectre Flare can avoid this? I'm not sure, just a suggestion