This can happen if the code operates on a moving mesh, has an outflow through boundaries, and no de-refinement active. The reason is that while gas is flowing out, the corresponding cells do not, but are ‘squished’ to the boundary. Eventually, these cells become so small that their timestep drops below the allowed value. Allowing derefinement should solve this problem, since tese boundary cells will then be simply merged with surrounding ones when they are no longer needed.