As a known bug of new update Legacy of Darkness, via crafting, now amulets can have slot. Below is an example.
As I said, this is a BUG. You cannot fill the slot of those amulets now and in the future. The developers from Korean have noticed in the next Patch notes (Memorial Sword) that this was a bug and fixed it (you can check it here – translated). All the rumors are crap, like slot of amulets can be filled with scrolls or those amulets are uber-unique. Don’t be get scammed by them!
And about slotted bike, GM Team of Cabal EU has announced about that and fixed it in recent patch. I’ll quote their announcement here too.
Bike extension bug fixed
It has come to our attention that it was possible to use a Slot Extender on a bike, to create a bike with an empty slot. This was a bug, and will be fixed with today’s maintenance. All affected users will have their items reset to a non-slot bike + slot extender automatically.