Especially when the bug very feasibly might not be in your system. In this case, Twilio is obviously calling out to a billing provider of some sort. I've run

Especially when the bug very feasibly might not be in your system. In this case, Twilio is obviously calling out to a billing provider of some sort. I've run into situations myself where the billing provider returns a status message equivalent to "failure" when it actually was successful. In that case, many billing systems will try again thereby double-billing or worse.