Samsung CB Pro on v.91 ChromeOS, Charging while running Monitor. Don't charge your Chromebook while trying to run the monitor. You can waste a day of hard work trying to find out why you Serial.println() are producing duplicate/overlapping output. The CB Pro charges via USB-C of which there are 2 ports.
Toshiba (2014) on v.87 ChromeOS. Charging while downloading a sketch. Don't charge you Chromebook while trying to download a sketch. You will/may get: "3020 timeout after 400ms" errors. Not every download but often enough to make you stop. The Toshiba CB charges via a separate charging port.
The above two problems went away when I stop charging my Chromebooks. Not conclusive proof but sufficient that I will avoid this in the future. YMMV