This is an example of what I call “sliver dropouts”;
thin slivers of the puzzle that are dropped-out (not included with the puzzle),
which together form a figure. The sliver drop-outs usually confuse the assembler
when he or she is looking for neighboring pieces. But even when the assembler
first realizes that there is a drop-out, he or she has no idea that there is a
larger figure formed by the sliver drop-outs until all of the surrounding pieces are in place.
Also, a figure formed by sliver drop-outs stands out much more than a figure formed
by an actual puzzle piece.
A close-up of the sliver dropouts: