Understanding the mechanisms by which crystal nuclei form is crucial for many phenomena such as gaining control over crystallization in glass-forming materials or accurately modeling rheological behavior of magma flows. The microscopic nature of such nuclei, however, makes their understanding extremely hard in experiments, while computer simulations have hitherto been hampered by short timescales and small system sizes. Here we use highly efficient graphics processing unit simulation techniques to address these challenges. The larger systems we access reveal a general nucleation mechanism in mixtures. In particular, we find that the supercooled liquid of a prized atomistic model glass former (Kob-Andersen model) is inherently unstable to crystallization, i.e., that nucleation is unavoidable on the structural relaxation timescale, for system sizes of 10 000 particles and larger. This is due to compositional fluctuations leading to regions composed of one species that are larger than the critical nucleus of that species, which rapidly crystallize. We argue that this mechanism provides a minimum rate of nucleation in mixtures in general, and show that the same mechanism pertains to the metallic glass former copper zirconium (CuZr).