When do mucus films plug lung airways? Using reduced-order simulations of a large ensemble of randomly perturbed films, we show that the answer is not determined by just the film's volume. While very thin films always stay open and very thick films always plug, we find a range of intermediate films for which plugging is uncertain. The fastest-growing linear mode of the Rayleigh-Plateau instability ensures that the film's volume is divided among multiple humps. However, the nonlinear growth of these humps can occur unevenly, due to spontaneous axial sliding—a hump that slides rapidly can sweep up a disproportionate share of the film's volume and so form a plug. This sliding-induced plugging is robust and prevails with or without gravitational and ciliary transport.

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• Flow instability
• Low-dimensional models
• Pattern formation
• Pulmonary fluid mechanics
• Thin fluid films