Unexpected extra characters appeared in the IDE Serial Monitor when write to Serial in loop()

[Silver-Fang]

void setup() {
  Serial.begin(9600);
  Serial.setTimeout(65535);
}
void loop() {
  Serial.println("Snapshot");
  char C;
  Serial.readBytes(&C,1);
}

Upload this sketch in Arduino IDE and open the Serial Monitor. There is a certain probability that unexpected random characters will be generated before normal output.

This is not likely an IDE problem because the same code uploaded to Mega2560 does not have this problem.