Overnight / Long time playing
Leaving your device on patrol or gpx overnight (or for any long period of time with no interaction) is not recommended.
It may lead Niantic to believe you are a bot account, and cause you to be 'CAPTCHA flagged'.
You will have to enter random codes into the app to prove you are really a human.
This is extremely annoying and also puts your account under closer scrutiny from the big N.