On holiday to Japan about five years ago, I decided to get one of those local 4G boxes that creates a wifi hotspot for your phone. However, as I had to kill some time at the airport before I found the place to buy said magical box, I thought I'd make use of my UK provider's kind offer of 25mb of data for £15.
I mean, it sounds a bit much, but after a 12-hour, £400, flight you don't care. I figured "it'll be handy. Why would they even offer the service if it's useless?"
Turned the phone on and by the time it'd finished reconnecting to the network and catching up on notifications, 20mb was gone.
...so I switched it back off and went to find lunch.