The Hercules-Corona Borealis Great Wall is the largest structure ever proposed: a concentration of gamma-ray bursts, and by inference the galaxies that host them, spanning on the order of 10 billion light-years across the northern sky at redshifts near z of 2 (Horvath et al. 2014). If real, it dwarfs every other known structure, exceeding the Sloan Great Wall by several times and the conventional homogeneity scale by more than an order of magnitude.
The challenge to ΛCDM is correspondingly extreme. Hierarchical growth cannot assemble coherence across a distance that light itself has not had time to span since the structure's epoch; inflationary initial conditions provide no organization at that scale to inherit; and the cosmological principle, already crowded by the gigaparsec structures below it, is violated outright by a coherent object occupying a substantial fraction of the observable universe. The standard responses are to question the structure's reality, since gamma-ray bursts are sparse tracers and clustering algorithms can manufacture patterns from small samples, a legitimate concern that keeps the HCB Wall's status contested in a way the optically mapped walls are not (Clowes et al. 2013 faced parallel arguments for the Huge Large Quasar Group).
The standing is therefore double: if the wall is an artifact, the tension dissolves; if it is real, no version of standard structure formation survives contact with it. Growing GRB catalogs from Swift and Fermi, and eventually THESEUS-class missions, will decide which.