I haven't re-read the article, but if I recall correctly the Redis in-memory data was being used to keep a working copy of the current account balance (not the canonical accounting/bookkeeping version) and it was the results of calculations based on the Redis stored data that was triggering the payment processing. So I think it _was_ a problem with a "metering usage" scale system, which cascaded into the payment processing system working as intended, but with erroneous data.