  The Graph Minor Theorem of Robertson and Seymour implies a finite set of
obstructions for any minor closed graph property. We show that there are only
three obstructions to knotless embedding of size 23, which is far fewer than
the 92 of size 22 and the hundreds known to exist at larger sizes. We describe
several other topological properties whose obstruction set demonstrates a
similar dip at small size. For order ten graphs, we classify the 35
obstructions to knotless embedding and the 49 maximal knotless graphs.
