In the Store, you can buy some Icons from the previous Worlds (2015 and 2016) that have been cancelled :
(https://imgur.com/opaQI46 ) .
If you buy one of them (despite the Error message) and wear it, the Store will not remove them but you still get a blank icon in your list :
(https://imgur.com/ZbdlsoG ) .
The problem is that you can have an amount of blank icons due to the fact they are not removed from the store (I have not tested the limit yet).
In-game and in your Profile, they appear as the Bot Icon :
How to reproduce the bug:
1- Go to the Store and buy one of the "Invisible" Worlds Icon with RP (remember his name if possible).
2- Back to the News and equip the icon (it appears as an empty slot).
3- Return into the Store and buy it again.
4- Repeat the steps 1;2 and 3 as much as you want.
[Sorry for the English]
One Grab One Ace