To refute Sconibulus's answer, I drafted a second puzzle with identical rules.
7 months ago, I released a collection of variations of an original puzzle called Trapezoids, but never mentioned that the cover itself is also a puzzle that resolves to a unique solution if a final rule can be discovered.
Shade some cells such that they form edge-connected clusters of 3 (trapezoids). A trapezoid may not share an edge with another trapezoid. All remaining unshaded cells are connected edge-to-edge.
Dots on the border between two cells indicate that one cell is shaded and the other is not.
Trapezoids cannot cross interior borders (bold lines). The interior borders create regions, each of which must have at least one shaded cell. Interior borders do not block unshaded cells from connecting.