Star of attraction
This Fe/Ni complex (1), whose crystals were twinned, posed a particularly challenging problem and is the focus of an exciting study.  The challenge stemmed from the pseudo-symmetry along with the absence of a detailed published X-ray tutorial that would help a crystallographer tackle such a case of pseudo-merohedral twinning.  The high-quality data ultimately allowed us to identify and resolve the twinning/symmetry problem and to correctly refine the structure.  Although this structure was not particularly difficult to solve, it proved to be very difficult to refine due to the ambiguity of the correct Bravais lattice type and space group caused by near-perfect pseudo-merohedral twinning combined with pseudo-symmetry.  To demonstrate to the non-experts in twinning how such a refinement could be handled the structural determination of 1 was scrupulously described in the paper referenced below.

Interesting fact
During the design of the Basílica i Temple Expiatori de la Sagrada Família in Barcelona, architect Antoni Gaudí found inspiration, among other things, in twinned crystals.  Whereas twinned crystals can be visually stunning, practical aspects of their structural characterization are frequently less fascinating.

Definitions
According to the International Tables for Crystallography, "a twin consists of two or more single crystals of the same species but in different orientations, its twin components".  In merohedral twins the twin law is a symmetry operator of the crystal system, but not of the point group of the crystal (possible twin operations for twins by merohedry are listed in the International Tables for Crystallography).  In non-merohedral twins the twin operation belongs neither to the crystal class of the structure nor to the metric symmetry of the unit cell.  In pseudo-merohedral twins the twin law belongs to a higher crystal system than the structure.

Reference
The study is described in the paper "Detailed example of the identification and crystallographic analysis of a pseudo-merohedrally twinned crystal"" by Ilia Guzei, Regine Herbst-Irmer, Apollinaire Munyanezac, and James Darkwa. Reference: Acta Cryst. B (2012). B68, 150–157.