Version: Arduino IDE=1.8.0; Mac=10.10.5; MCU=Atmega32U4
Issue: Arduino Micro USB Serial is crashed if send more than 255 data once to Serial. when it is occurred , the USB register is: 1) UEINTX = 0b 10010100 2)UEBCLX = 0b 0. That means the RX FIFO is empty but FIFOCON is not released, so the host has to wait until timeout.
Code:

void setup() {
  Serial.begin(115200);
}

void loop() {
  if (Serial.available()) {      // If anything comes in Serial (USB),
    Serial.write(Serial.read());   // read it and send it out Serial1 (pins 0 & 1)
  }
}

Send data:
12345678901234567890123456789012345678901234567890
22345678901234567890123456789012345678901234567890
32345678901234567890123456789012345678901234567890
42345678901234567890123456789012345678901234567890
523456789012345678901234567890123456789012345678