Specified flow through/out of junction X is above sonic flow and cannot be supplied

Full Message text is:

Specified flow through (out of) Junction X is above sonic flow and cannot be supplied. Please review your results and change your model.

This critical warning message means the user has specified an outflow which is physically impossible because of sonic choking. This message can also occur if Assigned Flow junctions, Flow Control Valves, or Compressor/Fans modeled as a fixed flowrate are used.