I accidentally left mobile data roaming on, as I landed in Malaysia en route to Australia.
Apparently, my phone managed to max out my roaming data spend before I noticed and switched it off.
My telco helpfully disabled my account before I spent all the money in the universe on data. Now, I just need to figure out how to re-enable it...
Contacting support is tricky when you are 10.5 hours in the future.