Structure blocks outside of the world border can be activated.

1. Create a flat world with a small world border.

2. Make a self-duplicating structure, like this (see attachments).

3. See that even through the border, the structure mechanism is continuing. This is inconsistent with other block-changing blocks such as pistons, which don't extend/retract if they are outside of the border.

But command blocks work outside of the border too, so this might be intended? On the other hand, I thought one of the reasons for the world border was to prevent infinitely-travelling mechanisms such as slime-block piston carriers, and this.