16 A appliance into 30 A outlet
The problem is that the outlet is capable of delivering 30A without tripping. The wiring in your 16A device may not be equipped to handle a 30A surge, and could therefore sustain damage because the electrical service can deliver more amperage in a failure mode. Or worse, if your power cable isn't up to the task of managing the heat of a sustained surge, it could cause a fire hazard.