Place down a double chest, place an item frame on the front of one half. Step back so there is a gap of three blocks between the block you are standing on and the chest.

Put an armor piece in your hand (Tested using diamond boots and diamond chestplate), and right click on the chest to open it.

Exit out of the GUI, and then right click on the item frame, the armor will be equipped, but no event at all will be fired. I have tested this using a utility to listen to every single event being called, and no relevant events are called at all.

This bug is likely less specific than detailed, but these are reproducible steps.

Note: On 1.15.2 this calls an PlayerAnimationEvent, but this is because 1.15.2 swings the arm for interactions. This is not the only intended event as without opening the chest, PlayerInteractEvent/PlayerInteractEntityEvent are called.

Why Major?

This bug can be used to equip armor the player may otherwise be blocked from using, or prevent plugins from detecting that armor has been equipped (I believe spigot should have an armorequipevent anyway, but this is beside the point). This is can cause huge issues for servers who need reliable armor equip detection.

Thanks.