Twilio experienced a billing system incident on July 18, 2013, affecting approximately 1.4% of its customers in various ways, including erroneous credit card charges, account suspensions, and delayed usage reports. The root cause was identified as a misdiagnosed issue with the redis-master process, which led to extreme load, performance degradation, and timeouts from slaves to master. The incident highlighted critical flaws in Twilio's auto-recharge system and its reliance on a single Redis cluster for in-memory account balances. To prevent similar incidents, Twilio has replaced the original Redis cluster, corrected configuration errors, introduced robust fail-safes, and updated the billing system to validate against double-bookkeeping databases in real-time.