One example that got me recently - opening the software keyboard on an Android mobile device changes the viewport height of a web page. Ordinarily that shouldn't really be a problem, but on some devices with smaller screens the keyboard takes up more than 50% of the screen, so the device orientation will switch from portrait to landscape (the browser doesn't use the accelerometer for that check). Consequently any media query of 'orientation: portrait' will stop being applied.
I had fun debugging that one.