This week a long-time client called with an issue – people were trying to buy items from his website, but the transactions were not being processed correctly. Credit cards were being charged, but the site was not seeing the successful purchase.

After some investigation, it turned out the payment processor had ever so slightly modified the return value for transactions, adding in a newline to the beginning of the value. This broke the code that was looking for the success code, thus causing the problems.

The key to identifying this issue was good logging, and reviewing the code carefully. Once fixed, new transactions worked as normal. It’s not clear to me that the payment processor communicated this change at all, and perhaps it was an unintended side-effect of another change they made.

When troubleshooting new issues to code that has been working for a long time, my first question is “what changed?” – if I can figure out where the change occurred, it’s then easier to rectify the problem.