create an instance of the custom block and put any block in the second slot
edit the custom block and change the type of the first slot
drag the contents of the second slot into the palette
The result is a block hovering at a fixed point in the palette (but only in that category) which cannot be deleted or dragged, but can otherwise be interacted with like a normal block in the scripts area (click to run, script pick, duplicate, etc.). Changing the slot type a second time seems to get rid of it.
Reproduction steps:
The result is a block hovering at a fixed point in the palette (but only in that category) which cannot be deleted or dragged, but can otherwise be interacted with like a normal block in the scripts area (click to run, script pick, duplicate, etc.). Changing the slot type a second time seems to get rid of it.