Abstract The discovery around the turn of the millennium of a population of very massive (M⋆ > 2 × 106 M⊙) compact stellar systems (CSS) with physical properties (radius, velocity dispersion, stellar mass etc.) that are intermediate between those of the classical globular cluster (GC) population and galaxies led to questions about their exact nature. Recently a consensus has emerged that these objects, usually called ultracompact dwarfs (UCDs), are a mass-dependent mixture of high-mass star clusters and remnant nuclei of tidally disrupted galaxies. The existence of genuine star clusters with stellar masses >107 M⊙ naturally leads to questions about the upper mass limit of the star cluster formation process. In this work we compile a comprehensive catalogue of CSS, and reinforce the evidence that the true ancient star cluster population has a maximum mass of M⋆ ∼ 5 × 107 M⊙, corresponding to a stellar mass at birth of close to 108 M⊙. We then discuss several physical and statistical mechanisms potentially responsible for creating this limiting mass.