Specifically with smartphones- the built in controls to keep you from blowing through your cellular data plan don't apply when your wifi...is connected to a meted plan. So your device thinks "I'm on wi-fi, update all the things" yet the connection still "cellular".
It would be a sad day in tech when your device OS has to implement an "airplane-esq" mode for metered connections that drops all data-intensive usage and maybe only allows certain notifications through.