It isn’t but from reading into Arduinos documentation as best as I can understand it “This just happens” sometimes and you need to use the reset button.
It comes from the failure of the system to re-detect the com port after it is reset by starting and stopping the serial port.
According to Arduino this happens due to something potentially messing up the interrupt vectors, but we know this is not the case with Serine.
What I think happened is that Arduino recently updated something on how their upload works on the leonardo and causes fringe cases to fall under this timing/rediscovery window.
I do suspect it is driver related however. And I think uninstalling and reinstalling “might” be a solution.
It’s going to be frustrating as heck, because I suspect when @smallstorm gets a chance to send it in, I’ll test it and it will work just fine. This is the case in 80% of customer returns when there is a problem like this, I plug it in, it works fine and then send it back. the other 20% of the time I have to use the reset button.
I’ve never had to retire an Arduboy from service due to an inability to program. In the beginning times there were some cases where I had to reflash a bootloader, but now that is fixed in the factory.