See https://github.com/arduino/Arduino/issues/3614

To improve stability if the sketch overwrite the last SRAM location
(in this case the double tap doesn't work but at least the sketch
starts without problems)

Bootloader is activated by quickly double tapping to the reset button

For some reason the blocking read from the original USB-CDC driver
is not reliable. As a workaround it has been reimplemented using a
polling loop with the non-blocking read function that, instead, is
much more reliable.

Eventually the USB_Read_blocking(..) in cdc_enumerate.c will be fixed
in the future.