Many micro USB phone chargers give 5V DC and 1500 mA output at input 100-240 V AC at 50-60 Hz (these values are from a Sony EP 880 charger). What happens to the output when voltage rises above 240 and when it falls below 100? What happens when the input is not 50-60 Hz? What happens to the battery and it's circuitry ?
Usually those ranges are the optimum ranges, which also have an extra allowance or a safety factor(around 1.2-1.5x), say upto 300V. Beyond that exactly that will happen if you run it over the design limits like Roger said. It will probably burn the regulator or other internal components.
Most likely, it will burn the immediate component in contact with the input voltage, and sometimes careful design will have say a fuse/or some other safety design which will protect the rest of the circuit and components, and only burn the fuse.