"(A customer had — I kid you not — a lightning strike hit her computer during checkout, and as a consequence the JS callback fired 36 times. This resulted in 3

"(A customer had — I kid you not — a lightning strike hit her computer during checkout, and as a consequence the JS callback fired 36 times. This resulted in 36 transactions, which Stripe processed without complaint. Oops." I can't believe the good old 'computer was hit by a lightning bolt' wasn't in one of your test cases, Patrick! I mean it's so obvious :P More seriously, that is one of the wildest reasons for a bug I think I've ever heard.