I don't use Twilio, but this was an interesting write-up. After reading it, I can totally understand how important it is to have safe-guards and redundancies baked into user balance information. I like how they even mentioned (although a bit vaguely) how they plan to implement those additional protections [1].

[1] "We are now introducing robust fail-safes, so that if billing balances don’t exist or cannot be written, the system will not suspend accounts or charge credit cards. Finally, we will be updating the billing system to validate against our double-bookkeeping databases in real-time."