One of the earliest examples of a benchmarking database was curated by Pople and co-workers, during their efforts to develop new quantum chemical methods for accurate energy calculations. Their database, dubbed "G1", consisted of a mere 31 atomization energies of atoms, ions, and organic molecules. This database later evolved into the G2/97, G3/99, and G3/05 databases; the second incarnation of which was subsequently used to fit the empirical parameters of the popular B3LYP functional. Since then, many more advanced and extensive -compiled -benchmarking datasets have been conceived, to allow the development and assessment of new methods across a large variety of chemical problems, e.g., Database 2015B by Thrular and co-workers,, 12 MGCDB84 database by Head-Gordon and co-workers, and the GMTKN55 database by Grimme and co-workers. Despite the ever-expanding scope of modern benchmarking databases, it is not always clear to which extent the data points present in them are truly representative of the chemical spaces they are intended to describe. As previously noted by others, 14,15 the make-up of these datasets is often strongly biased, e.g., towards highly stable, easily accessible compounds for experimental datasets, and towards small, easily computable molecular systems for computational ones. Consequently, the transferability of exchange-correlation functional accuracy across chemical space is not inherently guaranteed, so a benchmarking study may lead to incorrect conclusions if no attention is paid to this issue. For example, it is not necessarily true that the functional approximation yielding the most accurate results on a specific small benchmarking set will also perform the best for the broader surrounding chemical space. Even worse, while a benchmarking study on a small set of chemical systems for a given type of application may give the impression that a given set of functional approximations is sufficiently accurate -indicating that it should be possible to extract clear chemical trends from data computed at the corresponding levels of theory -this assumption may not hold if (some of) these same functionals struggle with specific regions of the chemical space under consideration that were not part of the original benchmarking set.