Lesson learned the hard way: I never tested the subscription renewal "i will not promote"

This is one I still feel a bit stupid about. I was setting up the ecommerce side for a small product with a monthly plan, and I tested the checkout, I tested the emails, I tested the thank you page, and I did not test what happens the next month. The subscribe button worked fine on the surface. What it was doing underneath was a one time purchase with a label on it that said subscription. Nobody noticed for a while because the first charge looked right. Then the renewals didn't come and I went digging, and there was nothing to renew, there was no subscription object anywhere, only a list of single orders. The fix wasn't clean. I couldn't turn those orders into subscriptions after the fact without charging cards people hadn't agreed to be charged on again, so I wrote a one off batch job that flagged every affected account and put a message in front of them the next month. Pay by hand this once, and set the subscription up again from your side. Some did. Some didn't. A few wrote in and asked why they were being asked to pay for something they thought was already recurring, which was a fair question and I didn't have a great answer. I'd done the same kind of integration before on other systems and I think that was the problem, I trusted the part I'd built before instead of the part I was building now. The renewal was the whole point of the product and it was the one thing I never ran end to end. Still don't know how many I lost to it. Anyone else shipped a payment flow where the first charge worked and the second never came?