A 30-amp breaker in a slot wired for a 20-amp breaker could result in the overheating of the wire, switches or other components, which could ignite a fire. A replacement breaker should be the same size breaker as the circuit was originally designed for.
A breaker protects a circuit from overheating by limiting the amount of amps that can be used. If more amps are being used, the breaker switch kicks off. Switches, outlets and wire are all rated for specific amperage usage. If a higher amperage is used, there is a serious danger of overheating, which could result in an electrical fire.